F I G U R E   8 .   S u b n e t t i n g   t h e   R o u t i n g   R e q u i re m e n t s   o f   t h e

I n t e r n e t

In Figure 8, a site with several logical networks uses subnet addressing

with a single /16 (Class B) network address. The router accepts all traffic

from the Internet addressed to network 130.5.0.0, and forwards traffic

to the interior subnetworks based on the third octet of the classful

address. The deployment of subnetting within the private network pro 

vides several benefits:

  The size of the global Internet routing table does not grow because

the site administrator does not need to obtain additional address space

and the routing advertisements for all of the subnets are combined

into a single routing table entry.

  The local administrator has the flexibility to deploy additional sub 

nets without obtaining a new network number from the Internet.

  Route flapping (that is, the rapid changing of routes) within the pri 

vate network does not affect the Internet routing table since Internet

routers do not know about the reachability of the individual subnets 

they just know about the reachability of the parent network number. 

Extended Network Prefix

Internet routers use only the network prefix of the destination address

to route traffic to a subnetted environment. Routers within the subnet 

ted environment use the extended network prefix to route traffic

between the individual subnets. The extended network prefix is com 

posed of the classful network prefix and the subnet number.

F I G U R E   9 .   E x t e n d e d   N e t w o r k   P re f i x

9