Prefixes and CIDR

A prefix describes a set of IP addresses that can be reached using the route. For example, the prefix 10.1.1.0/24 indicates all IP addresses whose first 24 bits contain the value 10.1.1. The term network is sometimes used instead of prefix to describe a set of addresses. To reduce confusion, this chapter restricts network to its more common usage, to refer to a physical structure of routers and links.

Prefixes are made possible by classless interdomain routing (CIDR). CIDR addresses have largely replaced the concept of classful addresses (such as Class A, Class B, and Class C) in the Internet. Classful addresses have an implicit, fixed-length mask corresponding to the predefined class boundaries. For example, 192.56.0.0 is a Class B address with an implicit (or natural) mask of 255.255.0.0.

CIDR uses network prefixes and explicit masks, represented by a prefix length, enabling network prefixes of arbitrary lengths. CIDR represents the sample address above as 192.56.0.0/16. The /16 indicates that the high-order 16 bits (the first 16 bits counting from left to right) in the address mask are all 1s.

CIDR enables you to aggregate multiple classful addresses into a single classless advertisement, reducing the number of advertisements that must be made to provide full access to all the addresses. Suppose an ISP has customers with the following addresses:

Without CIDR, the ISP has to advertise a route to each address, as shown in Figure 4.

Figure 4: Routing Without CIDR

With CIDR, the ISP can aggregate the routes as 192.168.128.0/17 and advertise a single address to that prefix, as shown in Figure 5.

Figure 5: Routing with CIDR

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