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space) based on a power of 2. Classless addressing is <strong>the</strong> most<br />

common model used <strong>to</strong>day with routers, and classful<br />

addressing is used with host computers. Classless <strong>network</strong><br />

masks are defined by a bit representation that indicates how<br />

many bits are used for <strong>the</strong> <strong>network</strong> address.<br />

The important thing <strong>to</strong> remember about classless and classful<br />

addressing is that <strong>the</strong> netmask representations change, but <strong>the</strong><br />

netmask itself stays <strong>the</strong> same. A 255.255.255.0 classful netmask is<br />

<strong>the</strong> same amount of binary 1s on <strong>the</strong> netmask field as <strong>the</strong> classless<br />

representation of /24. Both mean that 24 1s will be used <strong>to</strong> mask <strong>the</strong><br />

address space. The change is in how <strong>the</strong> IP address space is allocated<br />

and provisioned as variable address groups ra<strong>the</strong>r than as ridged<br />

classed subnets.<br />

These changes affect <strong>the</strong> way IP addresses are interpreted and <strong>the</strong><br />

way route entry for <strong>the</strong> <strong>network</strong> is created. With classful addressing,<br />

each classful <strong>network</strong> must have a routing table entry. With classless<br />

addressing, a <strong>network</strong> address group can be expressed as a single<br />

entry.<br />

Classless addressing enables <strong>the</strong> use of variable-length subnet masks<br />

(VLSM). VLSM is a tremendous win for routers and <strong>the</strong> way routing<br />

tables are created. With VLSM, you can variably subnet a major<br />

<strong>network</strong> address space or fur<strong>the</strong>r subnet an already subnetted<br />

address space. For example, an ISP assigns you four Class C address<br />

s paces, 192.119.17.0 <strong>to</strong> 192.119.20.0, and each <strong>network</strong> can<br />

support 254 hosts each. Instead of adding <strong>the</strong> four Class C <strong>network</strong>s<br />

<strong>to</strong> its routing tables, <strong>the</strong> ISP will just add one route entry:<br />

192.119.17.0 /22. Now, let's say you have nine <strong>network</strong>s spread out<br />

over three locations for which you have <strong>to</strong> provide addresses, and you<br />

are also using shared E<strong>the</strong>rnet repeaters, so <strong>the</strong> maximum amount of<br />

users you want <strong>to</strong> have per <strong>network</strong> is about 50 hosts. You subnet <strong>the</strong><br />

192.119.17.0 /22 in<strong>to</strong> <strong>network</strong> spaces you can use efficiently (as<br />

shown in Table 8.1).<br />

Table 8.1. VLSM Subnetting for <strong>the</strong> Network Example in Figure 8.5<br />

Network Address Netmask<br />

192.119.17.0 /26<br />

192.119.17.64 /26<br />

192.119.17.128 /26

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