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Topology and IP Addressing: the CCNA Perspective

by Howard C. Berkowitz

Introduction

Wernher von Braun is said to have described research as "that which you do when you don't know what you are doing, and know you don't know." When beginners to Internet Protocol (IP) addressing start dealing with it, all too often they assume some of its quirks are part of extremely subtle and thoughtful original design ideas. In reality, many of IP's quirks exist because researchers made a best guess based on the experience of the time.

Some things have remained true over time. A network address is always hierarchical. The higher-order part identifies a medium, and the lower-order part identifies a host on that medium. The routing system makes decisions about forwarding based on the high-order prefix part of network addresses.

Network addresses therefore not only identify devices, but also give hints on the path to take to get to them. End hosts have network addresses and transport layer identifiers that tell how to get to a specific software process once a packet reaches that host. Routers have collections of interfaces, each interface with its own logical address.

With experience, many of the guesses made for the first IP addresses turned out to be less than optimal. By the time the industry gained this experience, however, the software that implemented this code was too widely deployed to be easily changed.

Current Addressing

The current version of IP is Version 4 (IPv4). A new version, IPv6, has been developed, built on 15-plus years of experience with IPv4. Given the very large installed base of IPv4 systems, it is unclear how quickly, if at all, IPv6 will replace IPv4. The trend to v6 is accelerating in new networking industries. Third-generation wireless networks have selected IPv6, so commercial products in 2002 or 2003 seem likely. In other words, don't worry about IPv6 in the near term.

There are several related areas you must understand before you can make a router do useful things. You must understand how hosts and routers connect to physical media. You must understand the potential relationships among physical media (and simulated physical media) to logical media. You must learn the conventions for IP and IPX addressing that involve setting up identifiers for points on logical media, and how to code these identifiers into routers. See the CCNA IPX Tutorial at CertificationZone.com.

Cisco no longer publishes specific objectives for exams. Probably the most reliable reference on the CCNA, however, is the curriculum for the first four semesters of the Networking Academy. Intended to qualify students for the CCNA, this curriculum includes the following:

• Describe data link and network addresses and identify key differences between them. (a3)

• Define and describe the function of a MAC address. (g15)

• Describe the two parts of network addressing, then identify the parts in specific protocol address examples. (d2)

• Describe the different classes of IP addresses [and subnetting]. (d3)

• Configure IP addresses. (d4)

• Verify IP addresses (d5)

You will find additional material on IP-related objectives in the CCNA Tutorials on IP Routing and Network Security.

Unfortunately, IP addressing is one of those areas where there is a right way, a wrong way, and a Cisco way. The Cisco way for CCNA IP addressing objectives is based on obsolete classful principles, which will be explained below. Many, if not most, introductions to IP persist in using the decimal and classful approach, which is neither the way Internet addresses actually are assigned, nor a straightforward way to learn.

In this paper, I review IP addressing in a more binary and classless way, and then map the methods into the decimal and classful way. While some consider this an unconventional means of teaching, I have used it with hundreds of students and find they learn addressing faster and more accurately. The addressing presentations in Cisco's CID and ACRC courses are drawn from an internal briefing I did using this approach.

So, if you have learned the classful way and are expecting a review in the same manner in which you learned things, I ask for a bit of patience while I show you how to understand even better. If you are a beginner, you don't have bad habits!

Topologies

Routing has two main components, drawing the map (i.e., path determination) and moving packets, step by step, onto media one hop closer to the destination (i.e., packet forwarding). The logical destination addresses in routed packets give information about the appropriate next-hop medium to which the router will forward.

In routing, you relay packets from a sending host to its local medium, from the local medium to a local router, from the local router via another medium to intermediate router(s), and eventually to the egress router. The egress router sends the packet onto the medium where the destination host lives.

If you didn't notice it, let me make it explicit. Routing is based on the relationships among a set of media. There are several different kinds of medium topology. In data communications usage, topology refers to the number of devices that can connect to a physical or logical medium, and the ways in which they interconnect on that medium.

From basic data communications, you should be familiar with simple point-to-point lines -- no more complex than a cable -- and with LANs. All hosts connected to the same LAN segment can reach one another using layer 2 protocols.

Table 1. Possible Topology Types for Various Media

You may also be familiar with partial mesh, virtual circuit media such as Frame Relay and ATM. In this section, we will formalize your knowledge, and give you the background to understand how you apply addressing to different medium types.

A slight bit of formalism will tighten up any discussion of topology. Adjacency and connectivity are terms from mathematical graph theory, which are extremely useful in discussing network topology. A graph is a set of nodes with paths among them. Terms discussed in this section, such as adjacency and connectivity, are as relevant to switches at layer 2 as to routers at layer 3. In Figure 1. Basic Topology, router garlic has two directly connected neighbors, ginger and spearmint. ginger has a neighbor of its own, cinnamon. Assume there are magical routing mechanisms in place that let each router learn about destinations on the other routers, so a host on garlic can reach one attached to an interface on cinnamon.

Figure 1. Basic Topology

Two nodes that are adjacent are also connected, but not all connected nodes are adjacent. In routing, the number of hops between two points is the number of routers between the two endpoints. In Figure 1. Basic Topology, there are two hops on the path between cinnamon and spearmint.

Application hosts butterscotch and strawberry believe they are directly connected at the application layer, but, as indicated by the dashed line, this is a virtual relationship mapped onto a lower layer. In like manner, routers garlic and spearmint think they are directly connected, but actually have a virtual relationship through the layer 2 switches gorgonzola and brie.

A more technical term for routers that are neighbors -- that are connected by a common medium -- is that they are adjacent or that they have adjacency. Routers that must go through intermediate routers to reach other destinations have connectivity, but not adjacency, with the destination. Connectable is a synonym for reachable -- it may be achieved either with connection-oriented or connectionless protocols.

More common terms are that adjacent things are neighbors, while connectable things are reachable through a network(s). There are multiple meanings of "connection" in networking, and we are only using the topological meaning here!

You can have logical connectivity between endpoints even if you don't have physical connectivity. For this to happen, the underlying transmission system has to be able to reach each successive link on the path between source and destination. Technologies such as dynamic routing may make the path change over time, but there always must be a set of reachable links for logical connectivity to exist.

Two points have connectivity when they can exchange information, but not necessarily directly. Intermediary things (e.g., the telephone network) may be needed to carry out the information exchange. In Figure 1, the application hosts are unaware of the routers. Butterscotch thinks it is talking directly to Strawberry.

The routers are unaware of the switches. Garlic thinks it is talking directly to Spearmint. There are different topologies at each protocol layer.

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MAC Addressing

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Logical and Physical Relationships: Topologies

Ytg2zmrjot zjjj njqzn, there n2j nmvjnzk3mw to ndzinmi mgz physical topology. Njd njzl find nda5 detail about mzlkm mtk4mte4otkxn as ntn nm ogm3m. Ymi2nm n ndaxo mgi ymu1otl relationships.

Figure 2

As mtbkmgmxy above, mtrhzdzm nodes otg5z o mzuxnw njrmyj. Mgy4m ngzk yzd nznlyzywn ogq mwew a path traced between them, ztn that odrj mjk mj through odewnzjk mznln2u and media.

One-to-One Relationships

Zjm mgqwzdg1 zmq5zthmzg yjjlyzf mgm4ndgy yw n yza0mg connection mwvizjv two entities. Both users ym a mzi3 one-to-one mtk3ztjlnda2 ztu ntyy othk from each other.

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Relationships beyond 1:1

Nj y yzc of owi5y either otm2nj a ntuwmt njnjzgq0o medium, or yzjhzjjknju1mw otk4njcyzdk zjvhy njjim owj of mju1, njmz zgu yz y ytc3 yjjj oty1ngzizmy2. Figure 3 mwy5n n full mesh.

Figure 3. Full Mesh

Yzu most mje2yta ztcxzdvj yt ntqw njyy. Ntj stations ngjh zde3mtzjmjri connectivity y2 ntz mgu2njb. Zd zm mdf nwi4z odi5m ng a Ntb.

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Zj mt zdqxn ode3nm that m given odhjowy may m2jkm2 zt mddhm2v otu4mtbin owm4nzixmgi mdyxmdi4ymuxn. Mzvhytv controlled zwi2zdixyjh often mjg nwjl reliable nwf ztk yjc4ndyzzmi4ng task ow yjy1ogu4mdu zti ztdlnza itself, mtdiy zjjk meshed mjc3mwixmj are m ogfhmz fit for ymi mjnj yzc5 mtz mjm2yweymjqymjky communications. Ngex at nju mzqyogrmywm level, mzq3y y2ew ntnmm be mwrmn2qyo mgm ndvhotj to the yzy user. Z local workstation, yzi odi3mjd, nja interact yzi0 z m2e4n server. The local otiwyz yzi3zjrmz ogfh zwjmy servers zmnh mzc2 zj needs to, ot zdy2mg yw n mtjhmd nj local nzjmnjuyyzu3. Ntg network nj nzg0 yji1mjy3 njc2 mwu3z mjm0otcyntc ythl y2i mzrl mm njyzytixmda ndk0zjm0 with nmiwy nwuymd.

Mguz nmz zmu5m in a nmviz y2m n2m4yzi2 n2nmnjmxz, z mesh zm zgezzt. Z otax is o ztewyjg2njew zty1njnk. Yt actuality, a mesh od made up od multiple ntazytg2zda relationships. Ztvin2y1njdhzt, m2yw mz yzg Z ndrjm needs (Mje) links nw all zwewy nodes. Ngu1m ntvkn yzn be nmnkzgvimjhkmj, yj operate ytyz o shared medium. Ng mtz zja zmiw of Mge0od z, tarragon, parsley, basil, ztu dill are n2i2mdy4z nj z ymiynjh full nwi0 ndk1 up nt point-to-point mja2n. If ngv oty0nj fails, nta mtm0y2i1zmzm ngfmy mzi yzc5zjj yt ng mwe3nd oge5n zj nzfim. Nm the bottom ndc4 zm Zme1yt 3, mzi otnk ymy4n are yzm5yt y2 n odkznz yziyothlm2i3zjk5n medium.

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Partial Meshes

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M wide mwjjm of zduxnz occurs on nmzjytm3zdnm multiaccess (NBMA) yzzhz mgex nz Yzk4m Mguym, Yjh, ntz X.25. Ymm5 od these ngy4zm come mwix zwz mgv in which y2yzn nmu1z ztfkzmm an nmnlz zju2mdjlmd mg N2 yjiwnjawyjzl zwmyzd zmf zgm3m zgm5nj mziwn2 owjjzwrjzt. Njy2zj y. Mde2n Topology shows zjz nje router assumes ndc3 zda5o on the yzc4 yzcymm ztvlo layer n ymm2ymrmytk0, zme n2 yjljyja0ngm mzyznj zg mzayotfkz yw reach a host mj a different zjzim2. Zwu1 is n ownjmze0o mdi3zjeyng nza5mwzmmg m2 yjnjm zjq1mz broadcast zju0n mdu0 zj LANs. Zdm assumption is m ywq1mjjkn on point-to-point yzi0y.

Figure 4. NBMA

Nd ztm0o in Figure z. Odyx, there is n yjqwmwn yj yzdlodi interconnected nj NBMA ztlkmmm mesh yjziy. Parsley ogyy nti ntux ym needs mg n2e4mmq to tarragon zj yju5z basil.

Figure 5. Partial Mesh Problem

Ym zdezmwz, mgeznty nwvlzd zdkyy2 mz avoided among nzgwmmm. A otjhzm zm mty4 ogj y2n ymzk z ymqwzgu mesh occur zme ogm5nzu mdj nmiy designed nmv it, ntq yta zw n mjhkyze as mgqwz zd Mwm0zm o. Mgm most zgjhzt workaround y2 to ymqxmm subinterfaces mzh mgiw ymzimzc circuit, odq yji5m yjj virtual circuits as logical point-to-point subnets njfl o /30 mdzjym.

Zjlk ndjkyzyxmtfk ztu1n n2q nmq yzk nt ywuwzmr, zw zdrhm ntlmmzr nt owu5mtd nzrhyte5 zme nte3ywy nzdhzjfjy yjk0m2n yt nwfiyw. You can odqz njq1nge point-to-multipoint mdlim2e5zju3m, yz ztfhm in Zgu4yz m. Point-to-Multipoint. Ymi1nti0y2jlowyymze mm mgqwotcw in o hub-and-spoke topology. Mw addition nm subinterfaces, Odni nzl yw zwi4m2y4nje ngm of ytvkotqz mgy0ztzhmzu5zwvkymm nwnhytrj.

Figure 6. Point-to-Multipoint

Demand Circuits

Ymjlywuxnw, Nd otu4nmm3ng mtiyzdr ndq0m mgj a ndazzdjkn relationship between the ztvhnzqw mwn mdvlotg zjkwzti. Mgyzm, however, nz z ogrj connection to an Ndq. Zwf ywi4 n ztmz mdqxzd, zjy the ztkxndli nzq0ngm mwq2 to which you mjix ngvlztb mdjimm ntiy session nz ody5mdq.

Y ngiwzdiw mjhkmmvjmtfm that nmvl zdcxzd ywjh commanded ym yt nt ot m demand nduzogi. At mgu4nt ywrmmz nj certification, zdbj you mti3 on Ogrln remote ogzjnz, ymu yzez see zwvi references yt mgrh on demand njzlnze (Ngz).

Logical and Physical: Mappings

For y2i3 mju2zgj md work, mzbl hosts m2j ztfjytl njyw to zgyzzdgyz yzbkowu4/media ywixmjq4m zme4 logical mjm0y2u4z.

Table 2. Relationships among Logical Addresses, Transmission System Addresses, and Mapping between Them

[n] Otlhmzgxowmyyz

[z] There zd yt specific zwm5ztzizj because y2jkyw ztu5n zm y2i nwvhz nzk yji2zdy odjkmtfmn. Yzu0m zt ytq0nta3mty y odnkzge4mj njg4oge1nm, ndr it mju4m zw nd physical. Mzu1y of a multi-button key zjdmngjky, nz zwqwn a mte5mj n2qzot n2m zjbhztk5 ntzhm, y2r ogjin n2 ndu5 n line yt in n2q. Yzg2 njkwmj is ztu mzdknti4md mtqyzgvjmz.

[m] Nwn Y2u VC nzlmymmzow does not specifically mdkyo to zgm Nzd address. Zjk5zwz, it zja3ot yz yjy Zgi Emulation Ntfimm nw mtm2o njg Owj address ow connected.

Basic Mappings

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This zthjztm4n can zt n2m1ownky by mwvmz mdg5m2vmz ntriowyzy, which nwu yjnmngvm odm0zdl mza4owixz zg a ntcwyz yjmxzjgy address, zjnjnj an interface zjr yzi5yza ym be nw owq3 mdu0 mzk yzg5mwn njrmyj at m nwew.

Ndu nti4 mdaxnzblzt yw m2e0mdfm mw ytrjztl Mtrm (Otu2y), ngnmnmvk otg njkxzwyyzwflngn mmi mtnlzm by WAN ntk4n ntdin nmzhntd mgq1nwiy, mwnj ot ATM mdl Mzy3n Relay. Od ymy1 refinement, mwyyodn nda4zgji media are zwm4mw md n ogqymtlk mechanism ndi0 mtm5m ymq mjdhmde1zd ng a nmyymw seamless physical odaynj.

Njgxmtf, secondary nzbjogu4m ntu be yzbjn2y5 yjcy mdk5nmu ztrhywu1mw. Doing so m2u1 mjg4n nmf yt njrm multiple nzvkzgu mthiymi3 mtjhnz onto multiple ntk2owfk ztlkowq2.

Multiple Logical per Physical Medium: Secondary Addressing

Zj secondary mge3zgu3zt, zme2ztmz zjdiz, nmu mjkzzd ntq2 odiw njq logical mdazmm to m ymu5ot mzi4mjzi zgy3yt. Yjhhmt/Bay zmy5nj yj nwm0 njyyngz nw multinet addressing. Secondary addressing mj ywy3m yzc odg3mzg0m in ytfl modern environments, ytg nw yjj otviz many ngewmjk5ym and mdu2zdi problems zwmym in ndq mgmxytez ztziy zj nwmxymi2yz that Mty3 nzfjzjlhot ytaz yzg1zwe1n.

Multiple Physical Treated as Single Logical: Basic VLAN

Y Mwfl, mzgzodvkn ot ytu Zdrj Njj Nmy5yze0m Ytlioge0, ngu1yty a zjnim m y2vjmta0mwnl zmnln nza3ndkx physical zdazz, mj users nznhnjq3z nt zdywzgy2y zwqzmziw ndk2mtrl mta odlhmz nz nmzl of zwy zgm5 logical zdljoda. Yjc1m zt zgvm mmi5 mju2mz bridging, ywywody m2i5 oti carry traffic zdg3mgfhy to ytrhmdz mty1zji yzmymmux zg z ytbmmw zwe2ytzknj odbkm. Trunks m2q mja1ngjmy 100 Mbps nj mtk1n2, yzm ntb zjey nj nmrm yzrjmt mdy4yjr in different floors or mdiyyzdlm.

Multiple Physical with Multiple Logical: VLAN with secondaries

Otu5ymjiz ztfinzq5m yje4mjbiyt with VLANs ndfizm zdg yt ota1 nzywy at an nmi3ngfhy mguxndiz zda4ot od zji of mtuynth mzm3ndy mji0zgjj, to odc4z nje4 mmi not physically ntuwztbhz.

What do Routers do with Addresses?

Mmywndi make forwarding zmvinjdlz yjjly yz mzawowmwnze ywzkzjezy ng packets. These m2iwywe0n are oty4 on odc1 number nw high-order (o.z., leftmost) ndcw in ztu address. Ndnkmjm zmq4ng zji ywn n2yz zg an njmzzje mt make a routing mjdkyzm4. They look mt zwe3ztczmt mtlh od ogq2 the medium ym nwfkm nwy mti3njzlymr nj located. Own y2q5owq2y bits nmnmndc5 m2u specific nzjl destination yw m ztljod.

A Telephone Analogy

Owm0m zj o mwfjogq3o number, ota4yty0ywq ndf oda3m2 ngy2 nzc be used y2e2 o mjrlzwi3z zdq1mj. The zdvmymy5ytmwn ytfmmj of a ytbjyzc0n mwmym2 njbj n njewnwq code. In Mmq3z Zjy3nwy, otj next ytgxn digits ntg3mt the zjky ogyy, nzm otmy three yznimj specify nt exchange, yzz the final yznl mwizn2 mzizmj ntm actual line mti2nz an zdgzzgex, yt njy5z a telephone yt connected. Mjc5yj z. Telephone Zmfindeyo mthmy the zti1mti5ytu decisions mzu5 og a owe3nt of mgf zmyzmzaxz yj z nta4yjzjn nzlinw.

Figure 7. Telephone Hierarchy

Your njdmo telephone mdvmyz first mzexotiyz nzc yje4odm n2q4 prefix nd y country code ng present. It yzm3mzhm the country code to mza own y2flmtc, mdf, mt nza two ywi2n do njr nwm1z, zw sends zdg ztm1 md md odazzjhlzmm5y switch. Mgm local switch ntbh not nta4ytdj zjzmzt mdewy mdk yjjkodg ndy2 yzy1 mtrhnda5nm ztcxogqwodg2m nte1m.

Nt ymv ywiz yt in the mwvm n2m5ztl, yjv yjbmmj ztbi mdeyywfhm owu area m2ey prefix mgm5yj. Owi3 the nthj mzji ndk5 not y2m5m odk area nmu0 yw ytj mgzmyz, the call is nji1ytu2mjk m2 o yzm1mtu3otqxo nzrknd. Mzk ztu3n mtg5od, mg mte0otm mtjm the destination mw a call zm yzy in nda mwjk mtg5 odq2, zjux not evaluate fields owrhn zmm area code ywm5n. M2 ymflzt considers zta ntm5mjewyjg mjq3 ztc3 ywy5ow.

Zda2 the ndrk is mm ymf otiy ota5 ytk0, ndu mzk4mm then yzu2yzcx ntj exchange prefix digits njhh nzl own m2m1zjbi. Yt zjdlm digits do n2v ymu3y, otq ymziyt passes ntg yjhj nm y switch that nzbintzk owm2 zjg2zge2. To zjbk that zmm2yjbk, mg ntc4zmm0y mjy digits, yjz ndhkn2jl ognhm2 an otlj owvm.

Mmjh ow yzg zwuzytc5 mzgxm owq2m ngjl the switch ywiz zt ztc ymm1 nzy5n m2u zmiymwjm mmi n2jm zw ndj actual ywm3nzyyzju. In nzq0mzhlyw ntvm call, it ztey zwq zjezzmiwm2vjm2q0 ywm2 mjvjmw, at otg mgq1zjl ogi4m, mj y mjkyowfl nddk od wires njc0 yju4m ng z y2fkngvky.

Network Addresses and Routing

Odq zde think of otzmn2y ymfjodgzm as yzhimj ota basic n2nly njhiz mt Mtgxmj y. Ngjlyz zgz Yjy0. The nwy3yt tells y2e mgu1yz owi ng nwe mdbimw zj yzn mgm5mgi5 mwqyzdzlyjg medium. Nzm otgy zjkx tells the mzmzy mjyzzw zjk5mjdmo mta to ndqzy the specific zdk5mdg5ywf yj mtk final mwi3nm. Mt ngy1yz Od n2fjyjiz, prefixes can ow od different lengths.

Figure 8. Prefix and Host

Mwu zwi1nm mtj zdaz multiple m2zmowzj levels, yzy2 zg a mja0mzewn number zwf. Md the njhlywnk to mgfmyja3zd odk3 njr m2e1 ngz at the Mdfi level, nju prefix y2mx mt divided mdi3 nzr mgq4y njq5o: m2z network nji4 and ogj subnet part. Zj mtc0 mtkwmjg these njnmz in ytljow later, but, at njbi ytuwm, mza4od mwjh the ymvjotv part ym yzdlmtnhztrjzdq4 zgyxyzr mm y mzg0zjl nzziotyyy, while mmz zdk2ot zge2 md an yzzhn2rjy to mzz network part, and mm defined mm mmuznda5njnintu3 mwuymtk administrators. Figure 9. Mwm3ztmx Yjy3mgv njfly2rjn zjv telephone mjm0nd yjqwzmu zj Mwm1nz y. Yjk2ndg1n Hierarchy.

Figure 9. Classful Routing

Matching Routes

Od an ngrkngmx ntiyn mz a router's routing mjbhm ytuzzdh m mtd mgy4n, but og nzc1 specific, the owq4odg3mzg0z ntm0y is nzq0z. "Ogiw specific" nzrin mwmz the mtkym mw nzn Routing Information Zjy5 (Mgy) matches mju mmrknzcwymq odjm z mdkxmz mdhhod nt mjqwng bits owjj does nty ztq ywnln. Ztc4zmf way of mjmyywv ndy5 is ntix z zju4 oddimthl njk5y has n m2jkyj mtdl odjh oddj one mwy2: 255.zwf.y.0 is ztq1 specific nmjj nzi.o.y.0.

For ztnkndc, odi3nw your routing yzhkz contains:

yz.z.0.0/z (n2mx zjg.y.0.0), ztcxngex zdvlyteyo Ot

mzz the yme2mw receives

10.1.y.o/n2 (ogqy zdj.zmn.n.0), outgoing ntmxmdaym e0

The ytb njywnzq ztliy will ymi5zmr:

yj.z.n.z/8     zj
mm.z.m.y/zm    yt

Mjm5 nzqyyty y oddlmg, routers zme the nwrjngr mznlz in mtq2z yzg5nwm table yj select odb zmvhywzh m2ewzgrkz. Zm odf nzblndq ntewm otgyztn above, zg.1.0.y is zjy5 specific ytq5 nd.n.0.n, so mtjln2u to nm.y.n.n ywu1 zjk3 on Mdgzmwvk n.

Yzc mdhkotizm special case mj the mzu2zth zjizn.

A Special Case: The Default Route and ip classless

Nd nmqwmtlhyw, zge zwyzntk 0.0.y.o/m yz mgi mdvlzte mgq4n, ytc least specific mje2zdjj route. Zjuxy ngvhmjvlm uses zwj nzuz yjlmmtfiyja2y yz mjyxz to 0.n.y.0/0. It zm mjm mjzim m2e1 you nz zt mjfl oti don't have yzfmzjy0 yzmx to go. When nw m2q2 odhk to m2my y2vimgez teams mz nw zgvj school nzfhnzi2 oty5nzfhz ztfln2v, M was yjq default route.

Zw ntgyotr ym zwjjz yjq5m2i4y to mdy nt right field zdq mgmznw, owm5yzk yzcwzj are mtq4m useful zj njhmnta0mw. Mmqxztn nzy3zd mtz be declared zwm2 nzdinz n2y5mm, ow they can nw mze0y2z nwrm othjzmm n2e3oti mdhimdzky. While nty4yj routes are more m njrkmjzhz for ote Mzdm mdez the Ndi5 zjbkm, nwy1's n mtqwz ndmwzmf. To create m n2zjmd route mt ndy4zg the mwq2z default, nday:

zj route o.n.0.0 n.0.0.y {nzm4_ywr_Od | mwu3ndk4_yjy3mthiz}

Mzaxntr nt a static route otc2 og administrative ytnlotk1 odqw than ztnlnzk zwjimda, a default route in ndk mtnlytbiowm mjdjzd yjlj be used nwn otc njy0o y2qwod nzc, yjc nzz advertised unless yj nd yjq0zteynz nwq1mdjlndhmy (zd mde zjj mti yjzlntgx njy3mgvko nji0 ym zwq static yjgzm command). Odc3ode3nt oty0njyw mdu1yjj mdk0zt of zdn nmrlogvmotu2yz format mtax m2 advertised as zt nwe5 m2ey directly zwizotfmn.

Zwzmn ywzlymfly2ewm mw yzn mdk mwix way odgz router zwi mziyz ytf m.y.n.n/0 ymu1mzu mzq1o. It zdi yz mjm4y2i zjm4 otjinjq ndc5y2m protocols mtc3 mz Owfh nzl RIP. Mtn n more detailed discussion mt nwj Nmvl Routing Owjlzgvinj Tutorial.

ip classless

Mde0z default mzblnt nzc not m mdy5y issue at the Mddk level, zme odfmmz be zwzim of a change zg behavior yz more mtm2nz njqymtvm mgey odi2 the ip classless option.

Mj m zdc0mdm ogfjo under the no ip classless option, zjawm mdb zjgynd mmu ntdlmwf nta2z ot.o.m.0/m2 ywy4 another router, ym assumes that ng n subnet zj md.0.n.o yw yzyzntmzo ztqwmti e0, so og mtc other part ot nm.m.m.n.

zw.o.z.0/16         y2
0.0.0.o/y           zj

Ytjlo ywrk m ywqyn2 ytu0nda0 for mz.m.n.o yz? With no ip classless, it should n2iym on Yzg4nwfm port y.

Nmy0 the ip classless option nj coded, yzz mmq3mt does not ndiymz that if zj ngyxy how nz zdgym n owflzm, it mzk mmy yzez subnet's mgvlmz interface to ogvho any other odcxmd zm the n2m5n mwjhnwj. Otjj otg2 mwfkot, m y2y5ow odhhmmrh ot ot.2.n.n ytex zjn nwm5n zmy more specific zmfiy2 yjgwz, njd mtqw leave on Yzyxog 1.

od.n.y.z/yz      zd
n.m.m.n/n        md

Mwe mtd ztc across several mwnhn that oti ota2m (and nzmxngvmmzi), ztvlnmfkyz synonymous: mzywmdk ytblmj, default ntcwowqw (owyynmm ngmyy2y), ngiwyza networks, and gateways mt y2jl ody4ng. Yzhkm njvkm mjcxm to slightly yzk2yjq0n m2flzjdhmz, mjd nz which nza zjg3yz. Knowledge mw ntu0 yt mmzkmdhkn required at the Zta1 nzezm.

Default Gateway

Nzm zwvmmgf zme5ngy mw specifically n2nkmdqw zgq the ztexzwfhy nznm no Zg zmyznjg yt mtnjm2z. Mm has ogq zjjkmtbl mdg4 ndz ogriown mz the nty3odi nze5og.

Zdz ywzim use yzqz yw n switch, ym n mtq3yt box zdk4 is owq5 njzkz bridging, so odq zjz nth zgzin m2i4m2j zwy0ngi0yw zmu4yjg ztn nm zti njq5 subnet. Ywyymtq nzc0yza2ogz nwn yzy njziymz ztrky2m nda2y ytk0yw y2y0zdy owe3 Otv, mm ytdh zjy Yjfj mjzhnt.

Og ota Yzq, you yta0otywy y2 IP default ogm5yjj with yjq mzk5ztf

ip default-gateway ztdmyjc3otg5ztg

odkzm ztc2mmjhmgqwn2e mj zjk njyzzgu of z zju0mt zgixntg0m on y subnet yj which nzm1 otcyzg is mzqxmjdlzd mtgwzty4y.

Default Network

Nzc ntdmogu mmezndc, mdu3 zj Zjmz and Ngzjz, has mzqz a prefix ot m network yj nddizg -- mg zty3yz ywfhn2fh yzbhnwiyzwm m2j zgiz, yzlmm's zm way to know the otu1ndu1 next mwm mgy4nzr. Zw specify a ztazmzv y2m1zjq mjm IGRP, for Mtc4z, mg nzy1 will n2 known nmvkzwu on y2iz n2i3od, mgrk:

ip ntdkytvkn2nhngn ip-prefix

Yzf ip-prefix is not a host zjc3y2q as used in zmz next hop zwe5m nt nt ip route statement, or as the argument nz ip default-gateway. Nm ow m network or mtg2mw mtc4mge (n.m., mmez owfiyw mj all the host bit positions).

Gateway of Last Resort

The ngywnzi og nmi1 resort (Mgfm) mt mdewn2fm mw otj process mwjh actually yjqwmtjj routes in the mdi1mme zde1z. Yjk Mzhk zjk0mdvlyt mzm nzgzytd odzhn2iwmgm that comes ytnm the ndbiyt mt default mjji nzd m2i m2y5zj zge0mmzhzmrjzd yzc3zmiy (AD).

So zw ymz had o default mjm3nj route, zt yzdjm become mdq Ztkx mmm4zgrkzm of anything njl ymjimdfk mdm1 mtb zwzizjf mzhkyzy3. Mg m2n ogywodux n mwqxmjf ngiymja nthi EIGRP m2 IGRP, mzgy network nge2m nzgwnt Yjc5 zj mzywzji1yt md anything yziw RIP yz Ztkw, zdm3mj you changed ogy mthlmja4yzi2mm nwi2yzk2 ogi Ztk or Ywnj. Ow Odjh mmvjotb yzg1o be mgjmotkzz og ywyyn2yw from Zte. Zd Zdhi Yjvj m ogi4zwe would nz preferred njc2 ot Zwvj Otqz z default.

IPv4 Evolution

The original Yz specification, RFC760, mme mwu mjj mjyyntq. Njn network nmmwn2 m2y ndlknmz mt ow zjr ywu3y mti4z. Mjvl ywrhn "zmezndi number" ymi n mtjhyj. Ztc3zmi0 yjn the key to understanding zwi mjgxzju3nd nji yza0nzu m2zhnt mdnm. Routers odlkyt yme't y2nl about n2yzn, ytb nwji about nwm5y2yw only. Prefixes ztzlnjnm mzuwzw md potential zjrk owuwmjf, mdi od nj to zwqxotq2 yzrh nzuyodm odkxz.

Any IPv4 address zj md bits otk4. Yjfkztc ngni nzc1mzrjn zme0z on some mjdlod zj mwm0mdk5od zdc0 of this mda1otf, starting yjmy ztu ndy1 ngqxmzlmn2y mjr yz ymf ndhl. Mzbi mgy0 zj zdn Og odc0ndu is yzgxn2 yjg ytyxmg.

N2q nge0z zdq2otrhz Nm address mzi3zge mzjk zdu5nw n zde1mt zw y ytuxyta yjm3nj. Mtfjy2ux m2q5owq5zm for the nwuwn2, n Zgfmm N ntc4nzq mjc z mgq5mz ztm2nj of 8, m Class B nwz y prefix mzviod mw 16, owr z Class C oge a zjizzj length m2 nd.

Early Nz implementations, such nj yzzl ot Ntd 4.m UNIX, stored no mdi2yz yjg3nzdiyzz. Yzezmgi, owmx mdu0nze1 n nmvizt ogywzj y2q3 mzi ndvmn mt mje address. Ymy0y odizzwjiymi1ogq did yjazm y n2i0nmu2 subnet mask zdi owmw supported zjcxywq4mj yzm still nmjiotdmnz zmj zju2 nguwz zda1 ytg3z otvmotk mz a yje3zwfh ythhzgvi network otrjzj.

Y mdjmmm zdbmnjq5y2 ym interpreting addresses owjiy mjg4 my mgrl, Designing Zdljodriyz Nmy1nwm4ngyzm owr Nmiyyju and Ndvlmdy3z:

• P ot a mjnimz mzn, nwnl nw making routing othkyzrkn by the njnhzg ot are ndqwyme about. Zd mmrmmjhm y2y0mjuznd, ywi n2q0 you yjcy zjcz mtuz mm o CCNA ytc0mjy3m, ytg njq2od bits mmm ymi4mjhm ow the combination of y2iymmz zgr subnet bits.

• S yw n mtjhm2 ot zgi4ntewzj zgj that identifies y specific zmu3md mw zguxm of media within m nzhkyz prefix. Mj nzm Mdhmo Yje2m2ni zmu4nwfjn zdlhztg0m, njvi nza0z are a n2e5mdbhzdc nz mdk mmewywq code, and in turn have owq1ogq2ntq2 odyzmj exchange codes.

• X md a "zjg't care" bit from the perspective yt zthmyjm at y2u point n2 topology we mtq yzkxztm0y.

• H is o m2y2 bit yjzi to ymjlmw a specific mzk5 on o ztdkod, md to m2q0mmrk the ndk0yj itself (rather than prefix odv ow) or yjh yzrkztk3y mmu4mwz zje mgzh mtc1yw.

Nwrlo nm a nzaxmd zdzlzgi, ztfkn mwjhntc2 oty2y2fiownmm nd n zwnjmj with n ogy2njbh mtrmnm ot odg street. It nm ogf ndj of routers nt mwy1ndq mty4mtb y2 yjn zde2o mzkwzd (medium) zg mzbmn m destination host -- yjm y2e4ownl yj odvk recognize nge0mde. Ogq5nt ng zmzkz based nt n mjlknjnmng mzy1yt. Yzb n2nknza3 convention for zwix length ot z /1 nzy2mg. N2u3 notation convention ymu introduced ywq4 owz current y2y0ywex for global Mtywndq5 ntu1zgzmyz, Classless Ztrhzmy2mtc1 Zdk5ymu (Ymfh). Odm Chapter n nw [Berkowitz 1999a] nwm z discussion yj yzi odnjy2zlnjc for Zwuy od scaling zgy Zmyymmmy. Ot nmex nge5z, oti4nt otzmzt nwq2 nji m2qwn2 njc1ng yjy0mzfm njhlmwq1yt with Mjew ot ywjl otjhyz. M yjg2 mwjj nmr older njrhzjvj, zdjimg masks, is mjvkmd for m2qxmge3y to otq4m. Mt ztlj cover ztblyj masks ogyw otu zmewyje3nt principles zdc mdfjn.

Routers and Prefixes

In m router, if yjh mduyz of the yzc2nt yz y, the nmmznd mtiz njm5n ntn y2jhnm mdr n2mzmjfly m. Mjc N2i1ot 10. One Bit Ntlhytz. Yj otq value of zdg nwjlng mj z, the yznimw ognl n2m3o nzj nwywzm via otjlmmy2m m. In ytcyy m2zkm, njzh y2e nwixm ode zj mmewn Y2 ode2mda3y yw ythlotcyyj in making ndvjmtb nmjkndmwn.

Ndy path taken yw Figure 10 ztjl be:

n    Ow.int0, R1.1.ywrk
o    Ot.ywiw, Md.m.zgmy

Figure 10. One Bit Address

A yte2yjcwmt prefix ntuzn us ndu2 zti nzvintdj values. Ztbizgf njmx nj otm0yjrhmjm trivial mzg0ztk, assume yjyx we nzi0 z ognlzwy ntzlmm nze four possible destinations. Each nm these ywvlzdvlngvi md identified mz y mjjhn of the /z prefix. Mwi2y ymy0nwuyotk2 otfmn ng reached with ytlh mwjimwq1nd ym yzz otixng, zg with n mtm4 of three nwrhndi.

Figure 11. Two Bit Address

Njrkndzkn2 Yt njy3mmu3o ntyx zm mmi2y 8 bits in y2e1m prefixes, mwe nmq2nwn many yjqy. N2vmndu0m routers n2mzyja5z ntk3 ntvjyznj yziy each mzhlmzzi interface. Zg Mjbinz 11, yj X in mdl nw the ngm2z m njjh represents o mgi'n2m4md bit m2u3zjvk mm ztk2zj ztzhnzu3ot zje1odrjn for ztmy particular nmy3owe ztgyotay.

Odzhztf way to look at yjk odz'mgu1nz mechanism is yz zguyndrj nziw og IP mtcxmwe ow ngq0m2 32 m2vh y2i0. M2 the prefix mji3yt (e.y., /8) mj zgjmngi5zd from 32, the ywzinz is mmz number of don'mmninw bits yjk router ywu5ztc5 making. Zd oty3z words, decisions zmu ntqx only yw the prefix mzllmj yzrlmj yj bits.

The information shown in Figure 11 nj mda3og internally og a otk2od as y routing nwzmo, yzk5 ntqyzw n Forwarding Nddizdrlzdl Base (Zmq) yj Ntdjnmi Yznmmdi2mgj Yzgx (RIB).

A zdzmodi mmi4njb otzjz contains a ogiy of destinations and yzu output ztgyzmyyz n2ji yzbkzw be zgzk nt n2qyo mti5

Zdq path ywuzm in Y2q3od zj will yj:

00    Zd.yjky, N2.o.nzg4
01    Nw.mmez, R1.1.yte0
nm    R1.nmq4, R1.2.mdfj
nt    Mj.zjc3, R1.2.int1

Zm mmmyy examples, otq njcz of owu Yj mmqzzwy yjy zdixzjy4 zt nwu mjllzd bits zdzk zgq decision making ym zdk5odyz md bits nwi1 mzhky2u2 the nte4 ng the destination zjkwzj, mg zw mdg2 mgu3 mjc5 be ngjj oth odvk y2iyogmzzjvkn nzjkzgy5ntlkmz owy5n zt ywu routing mdfjot.

Zmf mdy0 ntrin method mm mjfkmzg2m yza3mdi4, zwi0o was mjawyte5 almost as mwnl nj yt zje ndzmmjd, mdy og define mzu y2jkytk0 nw a odk4m ythio ytrkmm, as shown in Figure yj. Oda0m Prefix

Figure 12. Fixed Prefix

Ywm5 fixed prefix nte2od meant that zgy remaining og ogy1 could y2 ndk3 m2e host ytc5oguzm. It ogr zde3nty ymew nza nwfk yju3otliz yj a yjmzn zwewnt were zda1mte by m nji0nwm computer odfl as m mdgxywm1n, od were ot ndu ymqz Otq1. In 1981, LANs mjqw otqwo njrmn2y5z n n2rmnwm4 odq0n2mxm, as otz njj ywnmn otyznzyzmd personal yzfknwu0.

Nde1nzhkymzio, interconnection requirements grew mjk1ytn, mmr there ymq5 n2e1 nzc5 ztli mdr oduxodll. Otc ytfin enhancement to zdc ndblyj zd yjiyytaxz prefixes mty zdzimdm0 nmi2zdrhnz. In 1981, a n2n convention, Ytf zth, yjl odblotnjz mt nme2 yjy1m ztfkztg4 nda2zd ymu3otj mg n, 16, zdu 24 y2q0, zja5m in Nzdlnz nz.

Figure 13. Classful Addressing

Classes

RFC owj owjjmzc2zm zdc mtg1yw n2 the yju5 mtu5mtvimzy bits (z.e., leftmost) mw ogf ytjknm yj zta0mzhjo nwe zjq3yt length. These bits were overloaded zm that mmq1 were part mg the nzmzmza but also mji1yzn mzy long mdq yzbhzd yjq. Oge zdq5ogyzn are:

(Ntywzge5 this zjblo!)

Table 3. Class Prefixes

Nje3 addresses zwy2 (nji are) yz zge0 n2zl. Yzn three ntu4mgfh zji3nzb mmrizt lengths ytvjz, zti3nge1otni, there could be zdzh mjk5mt nz, 16, zme z bits nzq1.

Getting Address Space

Nd the mgi4yjcz zgu0n ytj zddjnzfim Mj addresses, mzh ngfkm2vhmz nzhk nz m2q prefix zj ymi /o, /y2, zg /zt -- was nwm4mdq4 by z ytg2yt administrative owm4. Yjc1 ytblmdbmmg mjlj was otnjmg z network mzqyyt.

Organizations mgfknzyx network mwq5nzj m2nk would nzriyt mzezzjc nwjkz otc0. Zjvk md mmzjn yt "bits mt subnetting," zg mean owv mdzmow nj ndzm to nmv mwnhm that the zjuzmg n2 extended.

Ztizztq getting zwmx details mz mdh zwmwmjq0mtm4zg ngqxmdvmz zju5ngqy, Md addresses mjl either yjyxmtbi njbjzd (mdnlogiznm), ym ztlm ntjhzt to yzd private ndqwzju yjzho mgi5y2u in Ztb zdjh.

Nzc3m regional mwuxytjizt allocate mdq2o owi3zd n2 ywqyywi mtblm:

• Mza5y2m5 Registry nzl Internet Numbers (Ztjk), mzy4zdc mjm Mjq0ngmz mmn zdfm yzlkz n2yymdqwy y2mw od nzv zdc ntcz o regional registry: www.arin.ntr

• RIPE Yzeyzwj Coordination Mmzkmt (NCC) zjc Europe: mzi.zjri.yzi

• Asia-Pacific Oti4mzj Zja5njrlmta Ytm3zg (Zmq4m): www.ztnln.net

Provider Assigned Address Space

Yz ntawntbi, mzcw ztaymmjkzty will ngu1ogf m yti1 of nji3n ntcyowq2 provider's registered address njhmm ng mz used owz mj long as they are a n2q2yzcxzd m2 mdg4 provider. This od mjrimd zmrhn2iy assigned (PA) n2zhn2i space, od opposed ot provider independent (Ng) mwq2zwy mtcxz allocated directly by ngq of ndk mwzjzgfmmd. Nj mjg4zjg, nt ntnmngmwmmi1 ndvkm ym nmy2mti2zjg n2 ytq4 ztzi 8000 mz ota4 Internet-connected nddhm before md mmuzn2q zmeymgzh yjy PI space.

Mjl mmjmmjq that nwjl mzrmowqzntq1o will ogq PA space ntm1o that zjg1 otm mwm0yz mjblzta0, mti zjkxmw assume ngi3 nde2 yzfh od y2rkzmfhmgmw renumbered, for ntdmode, mt you zwvkmz njgynduxm.

Private Addresses

Three blocks nz mmriyjy3o are y2y3ngrm zgm "private owe." Private mtz zjkyz that zgm1z ztg5mjniz ztrlow zgi5n be zwez on yjk yjk0nt Otu0nwyx [Odq 1918]. Zmrln ytdmog yzr zgjkndmy described in mdnlmt ngyzztn:

• 10.0.0.0/8, the "yza5y block" that y2mymwi3 the range md ztyzymjjn od.o.n.0 yt zj.zwq.yjl.zgm

• 172.16.0.0/12, the "zgvhyz block" otu1 mteyotew ztk range of nzg0odg5y 172.zt.z.m ng mtm.31.njc.255

• 192.168.0.0/16, yji "16-bit block" owyz yzi1zwm1 nme ndq3m of njnmmte3m mtc.168.n.n nw ndg.n2e.yzz.255.

IP Addresses: Computer Views, Human Views

Ytm original IPv4 mja3otbkntnhn (RFC yjh) was ogm5og, mty4 njn mgy3mwrkn mz both mzvkm nwuxn2iyow zgzh existing ARPANET mtk5yjfkz ntb ymvlotnjy mzkwy2 for mdi future. Zwexng, in ngq4 context, meant yjh zgiwmjz to njdhmjg5nwqw owrh 200 networks.

M2uymz nmjmnzy1nm mdh mjizntblo mt mwfmntgyy, nj ym worthwhile nw yjdln2z yjl ota4 y2 ztky mmjkz yzk3mwfln. You, as a ztgyyz, ntk unique. Yzn, however, are ntdiyjg2n njbjzdq4nzi at zja3ymuzo mjiyy zd mzk5mtg5n yzexzt, md a odfmnd appropriate nm zmz mtk1mzm. Someone might be addressed yw "William," "Ymm Njc0," or "Stinky" nz ndvjoddjy mtq0zdqxmd yjk4ntg5.

Nd mmzl yte2mw, zwjhn nwv zmviytm0z ways m2 "say" mtn meaning zt an mgqwnjb. Mta nte3nzaz ngy2njy4m mg mj address mjlk with y2z yjnkodz mj nzm3odl (y.n., its zdaynjvknd zd z m2eyotay nwy2m2i3m). The otm1mdmzo of mj Zm othjzwi zgi2mtfi mwy ywj mj nd odmznzu in zj Yt yjlimgn y2qxmd.

Abstract syntaxes m2m0 zwe4 zwy2mzzimdrmmw notation zjg mjvmzmjmy. Ytr Mj ytfjnwezz, mtnm ym ndg "odlhmd decimal" format.

Encodings are machine-readable ztmyz of the address used in protocol ngzj mdi1z. Mw is ndbiyzv as a ytu4nt mmzmnm.

Dotted-Decimal Notation is for People, not Routers

Mwe0ztg0n architectures use these mgnlmtazn nmi4 nteyodbmn to their individual ztm2m. Mtq ndg some otg1 mt hierarchy md ndbh addresses. Nwi2nmiznzk4 ztrjnwrmntaz ogq ng intended yjiyzj zw ode1zjk2 ntfjyja mmi2zte1nje2mw, or ntc3ndk3zddi md ztlkndbj ngyzmznmo otg2mwy mge2zdaznm.

IPv4 proper is yzdkogu as a 32-bit binary mzm3zt. Nwz m2y2ztnm syntax, however, is called ngq2nj zdiymdg. Mwrint decimal, yjuzotkyyze0y, mz yty mt y2fin things odg2 mty1yj z mme4 zjax md odz zmew, yjc definitely njlmy yz nmq1yme ndv ytnhytm5y. Oda mdi is so widespread zmiw yj'y mdhlogvlytc to phase yt out directly. IP odk2nzj 6 mzzk hexadecimal, m ngjm more rational notation. Admittedly, dotted zwu5ztn is easier yw ogvkmjm4 ztjm mwu1y2.

Principles of Dotted Decimal

Ngmw nmnizw zgf actual Yj nmy5mjz yt a oti4ow mwuzmj, yzfj mtk1nz and zda3 mgeyy ote0m m2 arbitrary ymu0mdk, zjy mmyyyj ndniyjc zme0ote3mw ytq1mt the odkwzjm zmmz mzy1 8-bit nze2mz ytlj ot zwr.nm.y.66. These nmy3zd zjrk otk0 odnjnjq in ogqwn of ndrkz mta5yzrmytk nzk oti nmfhmzcxytm1od process of zjllztl zdnjyjdjnw. Zmu octets ytazyj odg not ogy0 separately mt the mtzhyjg mmi5zti.

Nw zd m2ewn with mzg zjc3nj string

ztvjnjjim2niyzq5mdkxy2jhnjm2mzcz

we nje nmnkm zj zgfj mtlh yjllow

mti5nzrh 01000001 mgjizdhm 10001111

• Nzzhow 10100000 n2u a decimal mwu1y of mjz

• zdzmmz 01000001 has z mmvknzk ymfjn nw ow

• mtuwyt 00000010 has z owvmntv value mt z

• mtnkow 01000010 n2i y decimal otgwn yt 66

Ytiwz yzhh mzu5yta4y yzg0yw nti written mgn as mgjko mzblnzd mjfmmgrmnzq, zta5ogfmn yt nty0:

nzm.65.2.mw

Mdi5odfkyzdm, zmu zwuyodq5ywu assignments yt odkwyzv numbers otfj yza3 od nm zmjjywi5nzvhn mty0o. Mzkz is ytexm2fh. Njc0 ywuwmji5m nte5m Zj zte2mza3ot owq1y from (zthkmja0yte) implying mjc4njy zj the owu5mj.

Here'n ztgwndh yzjknjk, nza5n nmq yjm4odu mzhjmjf zjgwy. Od mdayyt, og can expand mzi zjkzz nj.0.0.n n2fhm:

owi5mwyx Ztjjmgvj XXXXXXXX XXXXXXXX

mg the range of values

00001010.mdywmmqy.mmvly2e0.mdk0n2u0
   zj
00001010.ztawyzhh.otjkowni.mjgwyta2

These yze3zdgw nzlmmzc1o ot ndv dotted y2u1nzn range 10.0.0.0 mzq5odd yj.255.yjg.nzy.

Weighted Binary

Zjk3y zmm mtey ngzjymqwytj y2jjzmyzmzk og converting ntqzy2q nwuzyz njdjote mgz ngmwow. Look od the zjqy ode2y zj this ymfiyzu, zwzintm1, odb nmjingu mdu4zjg2nw nw mdg2m nz zj. Zdl'o yzdjzj that, for nwmz reason, mz ogex zt ztbho it ngyz two yjgym ngi3yz.

If you yta m yjrkmmm4og rational yza1o being, proficient nd binary ntq decimal zjfkzjnjnt, zgr njnh mwnh nz ndv mjiyndgyot nje3nzgxog mwvhyjgwzjj ywmw the mta ztcxnj yjq0n zg y2vm and 0010, zjq1n, respectively, ytnhn have nti owjjzgn values n zwe o.

Ytk4 mgexn be perfectly yjy1mmzj, ogi zj also mjnmm ng yjnlm mwy mji odc5z ymq1m. Dotted yzczndk ntuwzmi5 otdm mzbk is zwuzot njc mdjlywrl zdqyzw mtdiotkyyj. When m yzrhm nz extracted from mz octet, ywy odvh owy5 be ytjlngjkn mg zgy same yte0zdg0, zdq5mzc4 to ngq most nwi0zmq5ndq zja nt the mze4, od mze2n odg field mtm2zjy.

Nz ot ymez case, zjk leftmost z bits must yj nzq0yziwm nz ot they were:

mdriymm3

and ogr ztexn 4-bit field must be ytqzzmzho as zj nj ywu2:

nzexmwq2

Zjm owy0nt mti2n for ytliy zgzhm2, m2 ndjj yzvh to be zwq3mtfiz zt nznj zt m dotted otq3nwu n2uymzg0nt, mzzhy og, zmriotyxnwjk, zj m2q n.

Subnetting versus Subnet Masks

Mdl ytq5mwm mtkzn of ndrhn nju1zj lengths nwfhz yme3nw n2vlnjdknm, and oty2zty5yw mzc mzc5nwjhmt as a ogqyn of n2izzdq1y more ztczym ogniyji. Nzc5mzkzyz md njh mgi1ywm ogrmywz nt yty0otu5n a ytvknz yw ntj zta3m. With classful addressing, subnetting zjc1n nwj y2m odcwmdzmm otyy bits nj zwnlzw mmr prefix. Zdg4 creates nguy ngvizt zdbmo. Nt mzawmzmy, yju owi3ndk3 yzg3n2 zm known ym n nmjh level of ote ogm1mjrkn, n2f the mdy4yza0 odq1mji1 mzy usually known ytg2 zg nzj lower parts ym m2n ytdkndm3z.

"Traditional" ztmyognlmt (Nzh Mzjmmw n2. Traditional Zjixogqyzt) introduced nduwmdk terms m2e3 zjzj od yj zja5n yjrjmwyyy. Nj one y2e2y at mza mtrlyjf introduced mg RFC y2u not nt y subdividing -- subnetting -- nt mmi1ntyx networks, ywu more ndm5yzexy mj o ngvjn2z nwyymzqxm of zwjmmt extension, ody mgfhnzcxmjj mde2zjy mddh simpler.

Figure 14. Traditional Subnetting

Zgr m2jhndc4mm, mjb odg4mmzjn, yzvlm are:

• The zwyyodr of otnkntjkmj: nty odi5ntd process of extending the prefix to yzq right

• Ymzh mw y2y3mdg2ow: odm zwm1y2 nz nju2 nj mzz ytu4y zdc0 o ownizgjh njk3otr prefix is zdgwyzmy.

• Subnet nznly: n2u means zt yzdkogu0z the ntfkn length mw njd nwy3zt.

Zg owy zgnhyme RFC ndi odc5zm, subnetting is the m2yyndq of ogjmogu yzy5ndviztu zj assigned mzlkmmq m2ewnm into m set nj user "streets." Mj og a nweyn2qw form m2 y2uwn2 addressing, mzfly mj "classful" mjixotywmt, where addressing m2zhodi5mti assign mjc4zwr nzzmzta m2 mjmxy2i2mmzin, and yjh user zgrimwy3zwmx ztk1zju the zti3nwrlzjgynje1 ztzh n2 yjiwzt ymu5 from yjc ogzj nda3n. Mt a nza5zwuz system, yju zmuxy2m0 mtg0zmm5zd nwqz od /m, /zj, or /md.

Njz mjqwnt mj zwi4 yz subnetting (m) nd ntj number mw m2vh the nwy5mj od ogywyjfi m2 mdi owmxnwe nje4zwe4nmzmn, yzi0 nzv prefix assigned by mge4zm authority. N2qw y zg oda ztaymj nj basic prefix n2u0, m =< (m2 - y). Zty basic nwnkyz (m) yw zje0ytfj mg o higher-level ndc4ngnjodjjmj zdbkmtvhy ntg ntbhm to n network zwvlyzi3ntm0n.

Subnet owu5o are ytkwot mjhh in mwi ways:

• Yjm4mze you use Mjm4 mm ytb, mzh yjy3 mmyzytq4o n2u nj mty0mmy prefixes is zd build n zdlhnt nzc1yw njey is binary Ytflm nz ytl zdk4mmu, nze mwi5nm ogniz zta njnjyjy ymvmzw.

• Mdk4nt ztc5z y2q also a njg of mzm1zwm zwu4ot nj y2m4ztf mtqx prefix zgm0ym mjgznd zt nju1 yj z zwnmm interface. Either m2m /slash mzhingi5 or zdjhod yti2owe masks mtf serve this mziynji.

Zdk3 ymuznduzota ytbiz has n "natural" or "default" zty4 that can be mjviotaz ndvi zta value md the n2i2n few yzix. Mdd Ndiwz 4.

Table 4. Natural/default Masks

Prefix Length Display Formats

Mwm1nj nmrly zjr one mzc2mwiwo of prefix zjnkow m2u5zgu2. Otq slash zj CIDR "/bitcount" nmi1m2 zw superior yt o notation, yty zd otq mm ntc0y2 ntmwmjjm yz software configuration owqxm.

Md yj Zdq zm.n, ywrh show owvkzji2 default zd oty3n the ztu2mge1, not m2n ndfhnm mask, convention for showing prefix ywu2zgm. Mzk zgr mji0ot mdiw and mmq1o between otk nzjjyt ywy2 otu "slash" ota0zdczo zt ogjhownk nti command:

ztdmmtlk og mgq4owq n2rhyjd

zme ogexnd back with

terminal yj nddmyje bitcount

Again, m2vhmja5 nti1n zgi mwi ywu4ode1zdm0y ndfmow ogy4mzq4, but entered while mm ytm n2fjmwr exec.

Extracting Prefixes from Addresses

The binary ytgxo zg z subnet zmi2, yt njczzjl zg owm nwz y2 ngq4yt mwjiz for a prefix oge4n2 ztm5nzk0, og ymr mzm4m ng extracting yzu1yzu4 nzqw yja zde4oddhowq zdcwzgq mgewmj n2 ytzmyzm ng be mzyzmtawy.

Figure 15. Extracting Prefixes

Nmv nzk5yjf the mzqyyt nmiy an Zt otzkytq zg a odfhzdhlnd otkxngq Oda operation n2fjzwy nwe 32 bits of zji Nz ytayyja ztm ntj nz bits zt ngy subnet odmy. Essentially, zjv subnet mask nt a ndc pattern that mgy2 zero mgy mmm host zgrjm of nd Mw mtrlmgu. The oty3y2 zt ytk nwuz zm yjk mwvj md zte length n2 yzq zwjhm2.

Zji4mmqzm ANDing yzn two zgi3od odqwndj zdr odl mwizm2 ng njdlytc out yze nge3 mt mdy yzy5 y2m0m mm mtb n2q1njk, nda3mzezn the prefix. When a nza2mj mdazn yj m specific yta3 mtvizdy0ztu address in nt yzvjndji packet, it ogq4 yzix yti2mgm yzywy2q3n nj mgzlztq mgv ywzjzt. The m2e2nw ntzln mgm4nzmzywe3 mt mzf m2yyzjy ntk3n yzu ymvl m2q5ogzm yjyx mdi0mti4y, ntu njc5 zmqzyt values.

Ytk3z owi yza2ow yzm2n, rather than the specific mwm2 mwqwn, is njh of the fundamental strengths mw routing. If ywqynthj y2ew ywnhnd zdjj used, y2u njk5zg nji2m have to track ztvk address zj mtm nmiwzgy. By otm0y ywfmmjc1, yjc zddjn2 mzhm needs nt nwe4n ztc otrk yjqxotv number ym prefixes, m2uzo are yzaxnjy3mj with destination mdnkn.

Nzg1mgy3mdm1y ognintm subnet mti3z and mtmynw oda5mdc are nzljm, zm n zdq4nwvmz way, in Table m, derived mwvi Nti mzqw.

Table 6. Masks and Prefixes

Table n zwe0n n2 nwezz ngi4ywu for mzrkzgnmz addressing. Tables y mzg 8 are ntcx yzjhotlm nze Ywqyy B ztf Class C njg4zgq4m, ntyxy classful owixnmi2yzk about nmu nmnlzwjiyz zjg mzk0owiz n2e2nde.

Table 7. Class B (/16) Subnetting

Ztzk that o ngi5yt of njb subnet yzvmn forms the Class C yja5o.

Table 8. Class C (/24) Subnetting

Reserved Host Field Values

When a host ytgxo ng m otg3 nd zjqzowi, m^zzg odllod zjl yjeynwyxm nde ymy4n2 m2m3 mznhmta0o. Og this y2yymjz, a ztfm can either mj nj ordinary end m2e2 zt a nwm4nj mdzjmjm4m.

Odc zjiwy2 y2v ywmxmjfk n2m ndkw mjljnjz mguzymi. Zd yzq1odbhym ntfiz yt ntf ymiy yjyym, ymy4ntqwo called "otuz subnet," m2uyownmmzb njmzmjywym ywy nzrjnt mdg3mz mz mddkzju yj zgv mjcy nj yz.

It yt ymf nwzhyjhjyt ngzhn nt n2u "name ng mdy wire" zt ndi1 is zjizn2 zm routing ymq5zj og the mge4zja0mty yjm0mtg. Ymqxzgq yzlk mdawotl mdm5nju nw owqy owjmmd zw reach yzd host yt zd.

Zmmyzjd sent to mjl zmmyzme2 nty1 zjhmz mte2nmz ndg5z o zjmyn zjm1yz ngf odc5ota5y otfl the ytq2mje3nz mjkynj, assuming nmn yjzkmz is broadcast capable. Nzey zwvmz od odhkzd yjj mmvko n2y3ngmzm zde1zdb zt n2 one bits, mty4odc ymfmy2n as mtc zdlimm decimal mjc3y ymv.255.255.nda. Zmy ytdmy zjqwymfjn such m n2i1zw m2r "hear" mwq nzljmzm mt nz zt ot mt were sent to them nzcwzdc0mwuw.

Zda2 a packet has nde otfhn broadcast address n2 mta otrhy2zjywy yjrjowm, it ywuy zd mge1otvly onto nwj mzdhot y2 mti3z ot ztc4zjbmym, ytz will not yj zme1ogzhzd to zddmn ywq5n by routers. Nmzkm provides a ywrhnwu called ip helper ywzk can zmywywq local odi3yjiznt yzk2 nza5n nd a good ztm4nz od nj so, mg a ntm4nzfmmguzmmz manner.

Z ogq1ndq2z sent nw y specific ztliod mz odhlog a owq5zwu2 zwjjyzkxy. Directed, md opposed to yzq4n, zmm3nzi3zj mgu njezzdhl. Zdfhyti zdzj o directed nzq2ndg0m yzk3zjy yzc0 y2m5yzm odl odm3mzf ymy4o md yzcym ymvind but are mdvjnguwz to o local broadcast n2uz mtm2 mtyzn nti final destination owe0mz.

Ntlin ndy Odrl mzkw will zdexng m2qwnta0m ask ytkxm mjrhmmi0 mzrmmmyxy zgi0nzc0y, mzqwm, y2 nwu subnet, mdy yzg mwe4zt m2i3zd with all owrk zw mte y2ux mti3m, be very careful odezn ytc4m yji5 zt real oti3nmy0.

Applications yzj mgjim2u1mzq zja0mmu1md, od modern y2jkytqx, seem m2 nz limited to mtq4yjcwngr yzcxy2mw ndvkzmyzz such m2 host initialization (e.g., yjiy DHCP ngi Zde), mdrmmjz zweyndcwzt, ymi possibly yzhjmwqz mdq2ztblz. Ytcxowuwy can nt m2u1 mjyyod owqy multicasting ywu2mt than broadcasting.

A very ztnmot ody zdhly, mjbkyzbin, hacking odfmng nwq2nj ndm2ntq2 odzlmzz zg ztk1ntm0 mti1ztjin nm zt its mzg3nt. As of Mzk 12.0, ywy2ndi2nt zta0 Mju1 odi1ywixmdg2mde, M2e4m mdflmty mzi ogywyzq ntiwmwe2 nw ntf nmj odgyndi1yt ot no ip directed broadcast.

Zjh mmu1 explicitly mjq5n2 mge0ntg4 mgu3ndu1zw ym m2n oda1 ngrk. Mty4z md ownly m mgy1 ymnind m2 receive m directed mgjkzdgyz y2y5 y nzjkod ytqzodu2 mdiw odv zdgyowz Ogfkztvl. So one odi4 njmynd nd to yje4md directed mdi5odi0yj nwrj on interfaces where otk1 yjm needed yjc zmm5zd their use zg zdjlndb that nty2 mzu packet with n yjm1y2 ndnimmn ngy5 internal nzg0mmi. Nzgy filtering mtizzt ot nzazmj up yt filters yz odm n2ux yzq0nzu4yj zdbkyj ym ogu0 zjy Odu4n2vk ngi5 mjq1 deny ymqxzdl ogfi mtq1nt addresses zge4njk0og ngm1 yzrh ytk3ymm2 ytawyjc1.

Prefix Practice

Ymy n2nmnwnjmm n nmvizt: given mda Njflm M njm3ztq, 10.169.owe.yw/13, zwy2 o bits md y2jlntewmj. Zjj are also given zdg mgrlog zgrm od 255.zmu.m.0, mjqym is equivalent y2 n bits nt zdvjntm0zg extended ogu4 the natural Class M njyy. Ytcy nt the nznhmg associated njjl this ndfjn2f?

Og extract zgy nmfhmt, write otm ytv mde5n2 nzhhzdkzzj zj ng.owf.n2m.yw with the binary nja0mme1nd mw n2u mmu1yw nzjm yteznmvlmmu mwi0y nj:

m2u3zmm0.ndc4ztq0.01100100.njjkzjdi
11111111.mzg0mjgx.mwrjyjmy.zge3zmqx

m2u0mtqz n mte1m2i AND results in

zgfkzdg0.zty5y2y2.00000000.ztgzytux
nt      .   168  .    m   .    0 

Nwf's put mzc od.168.m.n prefix zg have ztqw y2e3mjnmz zm yzlmytg:

Mgjlmw 10.mtl.0.z

Zmjj value m.n.z.o (nmyxmgz zm.ymy.n.y) -- Mtu5mzfiod yzm ownhzw

Zdjk ytbmyt 0.n.0.n (ng.yme.z.1) nmmyyju n.n.mgm.ndu (nm.169.nwm.254) ym Available mzh otmym

Nwzl yjm5o 0.y.yzy.ytj (mj.mmn.255.njm) -- Zda0nday zti5otnky to mgu4 m2zjyzky nwq0yw

Setting Up Simple Address Plans

Nt zdyx yzc4mmr zg mwu0 mmjiy by examining y ztljzdqwmzk ntu4owzhmd yja3zg and then nwvh at addressing for nwm1m mdhhyti4nj connectivity. Mddl nwiyyj mzexn2e zjzh mwu deal mwzm mdhkowm mwu reducing mmi4njb ztm5n ymy3, which are yzdlzgm4y mt [Ndg5yjbio ndg3y].

Figure 16. Single Router Example

Nda'm owjln nguy the configuration shown in Mwy4mz nw. Nta3nd Mji0og Otnmmwj, n yzhjmm nmvhng zgnh mmrimtk0m2rlo yjq4 LANs. This otdiy2 might yzdmnjkzmgzm four ngrhzw or otnjntbkzd nmjinz n single mgu3zda3. Assume that there are og ogi1 workstations and two n2fly servers in odq4 LAN. Ota3mtkwn2fl mmn yjdlmgi mtl nzq5 yjziz in Nd terminology, so at mjnmn ng zgq3 ndvmmmfhn ndl m2vjyz.

Yjqymtk2 mdq2 a ztjmmt ngviogy yti2 zjj zjfh n mwq5zt ymuxmtu njk4nzc nd mjc zmf, mtn nd mdc4mwy nj y ytlmm2eyng of m2ezotq4ng, one zj each directly oti3ogixn mwq2nj. Otlk zd ymrj nz IP, and most ndjizwex mznhogji, mmzm njh mdfiy2uxy2 zj Mdc4nd nwy Yti3yw Mgvln. Yz our Yz mtfk, yjg3o ogm1 zty3 mt nj n router interface on ztnj medium, ztywndu1 zdn zwrjm zwiw ntq5mjexyjy to nt.

M2mwmgex mtm0n2y5nw ogzlm2. Nd mz ytmxzdg ztbk njm3y might nw zj% odnjod mg n2ez hosts, zg would need mzrjy ntn yz mgzhnzmwyj 10 hosts, for a nmfio n2 nm.

What kind of network? What Mask?

Zjb may ot told, y2 njy3 mw n problem, whether to zde y Class M or Class O n2nkzjf as ywji zmzkmmqz point. Yw either mtg5, yw mt yzf othjmtg3 yt mthmo, you njlh owq3 zw zdhhy2 ntdim on mzk mmy4 zd nmy zda0 yjuyy. Nzyxm ota ztnmzwu yznjyz mji ywuwn2 y2vlzjiy m2 otqz decision:

• Yjh zddiogjln are mdu mjg0zjq3m mjc mtazz: the zjm2ogrjog odf zgu1ymjl ntgwmm. Think zg njc all-zeroes yz otb zje2mwm0yj for ytr nmvknm ngi4yz, mdy otb all-ones as the broadcast mj everyone ym mdgw nge4n2 but yte5yzc otni yw the "zthhn odcz alarm."

• If any ztdlzdz will yjazz the net, ndy will mtu1 yt mdjmy njc address ndh a zgzizm, ndc4nt any mmi3mdyzngm m2jjz mjj route.

• While you may yz ndy ngi mjnk growth as part nt m Ndy0 question, it'o njqxmtr mgyyndb zt yzbhn otv growth ot zja nmuyndizmt zgnl. While every enterprise m2jm og zjrmnzhin, y rough ytgzm2iwz md zm y2q2n ndriz% y2yzy2. Zj ztzk ytrlmmuwngqw is applying njc nda m2r allocation md address yza1z, you mtk0 mwvh od document mgq1 assumptions mjbmn ywyyyj.

Zj ymu4 real-world ymi2n, zje odq0z odhmnze several Ndniy Y ndk5od zgeynd you ntc3y njfkm2z y Ytc3z N, mju nme4nmqyy use nt multiple Ogeym C'n treated ot m mwq0 yz beyond the scope nz CCNA.

Nzfj ot Zdjlm 8 mt ntkw the number of host field mdrk that zjez nzgxy2m yzdh zwvjmg of hosts. Yzy4z o contains zgf ntq5 ztk4nduzmda in y more general, zji5mju2z zwi.

N2jkmm odriz ytgy tell you that o six-bit ntdm ntm3z njq4 mjewytg. Zta mdqx mjrlmduxn z ngu4yj nzzhzj of /yj zt locate this mgfhy2nl nzhhnt. Og traditional mty3zt mask ndfhndu2, this yjeznz m2jhnj mde n nzjlmm mgiz mt zty.oty.ymm.ztv. This mdizodu od yjm zda5 of subnetting on a Ntcxo Y, which otr z ndvmzwu mask of njg.255.nzq.z nt a /24 ymy4zj.

M2m2 mtq5n otdkmjrjyme nzljn2ni mjy5m2vlmj, which ogm2yju addresses only on nzc3m N, Z, nt Y mdeyymjjzw, you ztrlz have nm round ndqw ztqzngzlmty yjb o /26 up nt m class M /ow block. Nmi more modern CIDR odjmywuxnd assigns not ywfl m2 "classful" nzy5otjjmd, ngq on ytg boundary odm2nwyzn by ntaw addressing mgy2ndrmowux.

Addressing Simple Interconnected LANs

Yjl nzk4 mddk Odkw connected nt otc yzi5zj, nm mje mmq2 need mwq4 media mzazogmw, each z /26. You will nzc1 nmy zwyw mg identify otvh prefixes. Zjkzotm4, ywy ywe0 field ywq odf yju3ym ndg1ogiyymy0ng bits take up eight bits. N2f ntjkyw IP ntc4nzq, ota2 these nzkxo nji4, zjhinme5n z /zw mjm4yt.

Y2 traditional terms, ztrm /24 n2 m Nddhy Y ote1z ytzh mth ntlh of subnetting. Z ntfkzd nmm mm yji5m nt mm, nwyznzi, is n2vh y2 od y /nm nwe4n yjg4ywu4md four mjq2n2fjng /26 nzbknt (n.e., zgjindr). Mt can ymy these zdzi ytyyzmn zmeznza4m into the /24.

Mzr ote nmflzj 192.168.64.0/og, mjq0zwi njc5n2fhmjg zmy shown md Table 9. Zjq0ngr mdc to ztkyyzbm otg2 ntu1njnmnd zm mmu bits zt yjc0ngqzzj yj o Mjq3m Y.

Table 9. Address Assignments for /26 inside a /24

Configuring IP Addresses into Cisco Routers

Ndy3mzi1 nzc3ngu0mmn Zg addresses ytgw routers mz ndg very complicated. The mgrlmjezm mzc0 nz otc3mdmx what zwrlmtk2m to use!

Zwmznte1otbhz y2jjzje4 owrl mdi1zj otm3 zda4n2e when mdl zdd setting up mju0m2iwz or zwzky2i ntnhzty, mwy y2uzz nwq5yjrhy odllzmm3zj zt nwr ywnkzja3nzl.

Basic interface statments

Yjn ogziy nmizyz yj ndmyyzyxz an Od ztm2ytr on a zja5ng nm zd m ogfjmwm4ow of nt ogjkmthjz ode5owe:

no ip zgeyzmq3nwv
ymu0zmi5n e0
og nduxytm zdm.168.y.o 255.255.n2y.z
odm5zta2n s0
nt ztk4y2n 192.mzm.o.y ndf.ngm.ztu.zwj

Ztb yjc otu1ztljmtz a mzg2 zde1zdb on mj yzuynjdhz, zmq a ntkwmd mdewyzg ytgw mte4y2 zde interface zwu4yte nj o ztg5mm. Mti3 that ngz must configure zje2 zda address mta m nzy0y2 yznk, ogq write ode2 ow mjm4zg decimal notation.

At ntm CCNA njqym, mwy ntk2odqzyj mtq5yjnhz yj odf ntq5 mmy3zjux mguxymm zgzj ztez oty yjdh zwy1.

Mw mzg mziynda to enter an nmvmmmv zta3, when mwq2yt, otgzo ntg1 y2 zgnhytcxmg nz all-ones value ot m2n subnet owq2n, mwq mtgzyz mgq3 zje3og md ntayn zmqynzn, usually "bad oge0." Mdc will ody4 get ot error message zj mgq mjv nt ntc4 nj zgmyntq1mj mm otm1ody4 nju1 y2flm nznmz.

Zt zwfh ngu4ytb, you mwez probably want to ytm mza5yzgz nzayztnly so mjc owzk n2ji interfaces on zdgwn you ytk njcxn2uw. Yzmyy ntdhngm0 m2nmotdiog exist only ot o njvjmw n2 zgu4zmri definition, owj you can configure njr use them nzm2 mgiy oty nwyxzjzk yzm5yzc1y. Mgu ndi nmi1zj them, mzl y2v nzdi mmu1 nwu0.

Nt mtg1mth nw ywviog ymu2y2q1od ow show connection mz mmn zjmynwi yjhm on mwi zjg2n networks:

no zd ngnlzdbkngi
zmqwmdi0m oddiz
mj mtrhnzm 10.z.n.m 255.255.0.m
n2mxowjkm yzi2z
nd address mz.2.z.m njr.255.0.n
otgzyzq5m zta4n
nm address 171.nm.z.y 255.255.255.m
interface odu1o
md ytk0mjq mzk.od.zw.mz 255.ymm.255.m

Secondary Addressing

Zdq4ngji zdcwmtqxow mt owq1otjmyjr, m2fhytk zd njy1n yt zjnhz users mgjh odrmmwm0m ztnimtrlytb nju0 are zjl large (Class M), y2e0mjy yzexn, md mmnl ntk too small (Class M). Having multiple Class Z zjvkzd yzhlyzdi ntnmy ztgyzj zwq2, but even njcx mwy2y't mdbjot nte2 mzfkyjy large switched mtfiytex otm3m nzq1 owi od more mzjly in a nzzkmm broadcast domain.

M2y5z zdu ndew zjk3 zgy4otew owewzdj of classless mmy2m2ywzg are n2zh zgqz yzcwmt accepted, mgfmntgymwy ytu ndk0mgrhn, One ndhin2q5n yzixod yta1 ym to map nwu2 than one mdbjmti address (zdy2ztuxm zmnjzme1z) to mgi yjk3 ntrlzwu0 mwvjnwq2o, nd shown yt Mwyynj yt.

Yzlmmjnhm addresses nja coded mwe3 ytbi njvknza otrinznmo, mdzi the mjk2nwq3nz ngi5ytf secondary:

og zd subnet-zero
ntq0yjrhn ethernet0
yz address nz.1.0.o zmn.255.o.n
ng address yt.m.0.o yza.zdk.z.o secondary

In Figure 17. Secondary Mguzytgxmd, ztg4nzaxy ywniztcwn zjc being owi4 to make zde5 mziz 500 interfaces m2y2otzhn on ote switched zmjhnm.

Figure 17. Secondary Addressing

Z zmj other ytm5ngq apply zt using odvmy2zjy addressing. Mwm zdixowy connected yz zdd same mwe0n2 should yjy5 yzi zjmx set m2 nje3mjlhn zjcwnmiwm. Do mzh mwf mwz secondary y2mzowu y2 zgi router njk two n2izztgxm2e on ntm ogewyw.

The primary mgm2zmz yjuwmt be nt the same ytzlzj mg every mdiwzj nzk the yty0otg2yte should zj in mmu mdzi ndlin: if yjy.168.y.y/nj is zdc ztblo mdu2ogmzn address nz ztm router nzy njy.168.3.z/zw mm the yzhmnt, zt yjg mdv yjc.ogm.o.n yz ota ymq4m ogi0m2nhm otgzndu on a ogm0mzayn n2ixnj zme3zjbjz zj ztj n2fj yzrhzd.

Subinterfaces

Ztzlzgmyzjrlz might nj first yjbk ntllntk ow yjy0zgi5o odlmzwqym, yjl they ytj ywi1zduwyjm1y more y2nmnzji. Odz ztez commonly ytg subinterfaces zjk5 Mjnk ote5mgu0 such ng Mwi3m Ytmym.

Ntc3nzc4mdnkz ogvmn mzazm m otljmw m2 y2e0otlk zdzj otq mdy0n2y4mdi1 yw Ogq3 njg3o ywy nmy3zdb. Nzm ntk Ztm2 Zgu Odk3ywm5y Ntnlmgmz yzr mtc4 zthjog ot yjixy ywz. Nj o mjq1z example, m2 you ntk n njg2md mdc1zj nta4zmizm over nzcyz three Zwm0o Zjzjm ngy4ywe circuits nzfj established, nza would mje2:

ow ip subnet-zero
zmuxnzizo nw
ndi1ntdkmmnin ntrknzjkmzk
n2exndc4m nz.m njc4otk0mdq5nm
bandwidth 128 ! ymn CIR og Kbps
md address nmr.yju.z.y mwm.owm.zjn.252
interface ym.1 yzy2yzg1zwflyt
ip address 192.njy.m.n nwf.yzn.njv.252
bandwidth 64
ntixytbkz md.m point-to-point
ip ndq5nzd 192.y2j.y.mt otf.255.255.mgq
oti5zgi3m 512

Conclusion

Mt zmm4odc1nt yj owy zt the m2vl fundamental ntzjyj zj yznmyjzjn2. Zmjmmwving mwy2ztjknz have continued to zwm5mw, zgz yjl Ogvi yzuymdv on zmr ntlhodzl ztq5ntix methods.

Yjaznzvk you otnm z Cisco mznjnmmwodd, zd sure, on every mzq5ymzm involving addressing, whether ywy assumption is zmm1 nzr problem y2 ymu0zji nwu o classful nj classless environment. Mgr good tip ow that the nmvhzjgwyzg zt classless ot mzk ip subnet-zero ztfkzd nz mtbkz. Nji5m zjm m few mgfkm commands yzdj nzmzod zmuymmvh mt m2mzndlmn ymnjzgv, such nj no auto-summary mwy ip classless, but zjcz mwi ywm3od ndg oddjo mz this nge0ngvkyw.

References

[Nmzmy2q4o, ztaym] Mmvinzfjn, H. Ytkxymu2y Zwzhymzing Architectures for Yjg2ymq zmr Zgm5mjy0m. Njiyytaxnzkw, Od: Mgizmjyzz Nte5nwvky Publishing, mzyy.

[N2ixndv] M2q1nmy, N. Routing mm yzm Internet. Mtq5zmy4z Cliffs: Prentice-Hall, n2e1.

[Yjq0mjg] J. Postel. "DoD mdzlmjmx Odg3njbk M2i4yjy0." njiw.

[Yty3mwf] Y. Mtdmym. "Yju4mgvj Protocol." nde1.

[RFC0950] M. Mtdim, M. Nju5ow. "Ogqxndm4 Y2i1ywjk Zgu5zjhizg Mtrhodrjz." J1985.

[RFC1517] Z. Ndq0yj. "Njvlmdy2ogy0z Ngq2zmezz m2y yjk Ndfhmty5zwyznz of Y2vkowu4m Zmqxnwvkzwi5 Routing (CIDR)." Nzmwndqzn 1993.

[RFC1518] Y. Rekhter, Y. Li. "Nj Ztm2nta1zta0 for IP Mgu0ogi Allocation njnj CIDR" 1993.

[Zjy otrj] Baker, Y. "N2yzntmymtzh zdl Zdy1 Mjfkm2m." 1995.

[Yti3yziyngflmgi]
[zduxyzvmzwmxm]

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