Extended-Network-Prefix

Extended-Network-Prefix


Internet routers use only the network-prefix of the destination address to route traffic to a

subnetted environment. Routers within the subnetted environment use the extendednetwork-

prefix to route traffic between the individual subnets. The extended-networkprefix

is composed of the classful network-prefix and the subnet-number.

The extended-network-prefix has traditionally been identified by the subnet mask. For

example, if you have the /16 address of 130.5.0.0 and you want to use the entire third

octet to represent the subnet-number, you need to specify a subnet mask of

255.255.255.0. The bits in the subnet mask and the Internet address have a one-to-one

correspondence. The bits of the subnet mask are set to 1 if the system examining the

address should treat the corresponding bit in the IP address as part of the extendednetwork-

prefix. The bits in the mask are set to 0 if the system should treat the bit as part

of the host-number. This is illustrated if Figure 9.

The standards describing modern routing protocols often refer to the extended-networkprefix-

length rather than the subnet mask. The prefix length is equal to the number of

contiguous one-bits in the traditional subnet mask. This means that specifying the

network address 130.5.5.25 with a subnet mask of 255.255.255.0 can also be expressed

as 130.5.5.25/24. The / notation is more compact and easier to

understand than writing out the mask in its traditional dotted-decimal format. This is

illustrated in Figure 10.

However, it is important to note that modern routing protocols still carry the subnet

mask. There are no Internet standard routing protocols that have a one-byte field in their

header that contains the number of bits in the extended-network prefix. Rather, each

routing protocol is still required to carry the complete four-octet subnet mask.