CCIE Security Exam Certification Guide

Classless Interdomain Routing

Classless interdomain routing (CIDR) is a technique supported by BGP4 and based on route aggregation. CIDR allows routers to group routes together to reduce the quantity of routing information carried by the core routers. With CIDR, several IP networks appear to networks outside the group as a single, larger entity. With CIDR, IP addresses and their subnet masks are written as four octets, separated by periods, and followed by a forward slash and a two-digit number that represents the subnet mask. CIDR representation can be either a forward slash with a one-digit number or a forward slash with a two-digit number (for example, 131.108.1/24 or 131.0.0.0/8).

In the past few years, the expansion of the Internet has been ...