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Chapter 5: IP Addressing
Classful routers accept only a few prefix lengths:
Class A
20.0.0.0/8
Class B
172.10.0.0/16
Class C
192.10.10.0/24
Classless routers accept any prefix length:
Classless
31.2.1.0 /10
Classless
172.20.0.0/20
Classless
192.10.10.0/28
Figure 5-4 shows an example of networks that use classless routing. Note that each network
uses a different prefix length.
Figure 5-4
Classless Routing and Variable Prefix Lengths
Network Addressing Guidelines
The host's network and address must be unique. If the network is connected to the public
Internet, the network ID must be unique. If the network will not be used globally, the ID
must be unique within the local environment. The host address must be unique to the local
network ID.
The valid network ID cannot begin with the number 127. The number 127 is reserved for
loopback testing.
In the case of a Class C network, the first octet cannot be set to 0 or 255. The host ID cannot be
set to all 0s or all 1s. When the router sees a 0 address, it does not route the packet. When a
router sees a 255 address in the rightmost byte of the address, it treats it as a broadcast.
For protocols such as OSPF, EIGRP, and RIP v2, the router can be configured to support
classless routing. Classless routing allows the prefix to be increased from the fixed classful
length. Classless routing allows prefixes to be greater than the classful specifications of 8, 16,
131.108.1.0/24
131.108.13.8/30
131.108.13.4/30
131.108.0.0/16
131.108.2.0/24
87200333.book Page 150 Wednesday, August 22, 2001 2:37 PM