Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 277

Serial Line Subnetworks Table 6-12

Third Octet Fourth Octet Subnetwork

01001110 000000 00 172.16.78.0/30

01001110 000001 00 172.16.78.4/30

01001110 000010 00 172.16.78.8/30

01001110 000011 00 172.16.78.10/30

... ... ...

01001111 111101 00 172.16.79.244/30

01001111 111110 00 172.16.79.248/30

01001111 111111 00 172.16.79.252/30

Each /30 subnetwork includes the subnetwork number, two IP addresses, and a broadcast
address. Table 6-13 shows the bits for 172.16.78.8/30.

Addresses Within Subnetwork 172.16.78.8/30 Table 6-13

Binary Address IP Address Function

1010110 00010000 01001110 000010 00 172.16.78.8 Subnetwork

1010110 00010000 01001110 000010 01 172.16.78.9 IP address #1

1010110 00010000 01001110 000010 10 172.16.78.10 IP address #2

1010110 00010000 01001110 000010 11 172.16.78.11 Broadcast address

Classless Interdomain Routing (CIDR) and Address Aggregation
CIDR permits the address aggregation of classful networks. This is accomplished by using the
common bits between networks. The networks need to be continuous and have a common bit
boundary.

BGP version 4 supports CIDR, which permits the reduction of the size of routing tables by
creating aggregated routes that result in supernets. CIDR eliminates the concept of network
classes within BGP and supports the advertising of IP prefixes.

With CIDR, ISPs assign groups of Class C networks to enterprise customers. This eliminates
the problem of assigning too large of a network (Class B) or needing to assign multiple Class
C networks to a customer and needing to route each Class C network, which makes Internet
routing tables large. BGP is covered in Chapter 11, “Traffic Management.”