Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 83
Classless Inter-Domain Routing (CIDR)

There is a lot more to CIDR than what is presented here, but for our purposes, this will do. With CIDR, network numbers and classes of networks are no longer valid for routing purposes. This is where the network IP address format changes to <IP Address, prefix length>. Mind you, this is for the Internet routing tables (ISPs); Class addressing is continuing to be used in customer environments. Classless could operate in a customer environment, but most hosts would not understand this type of implementation. The millions and millions of hosts that are attached to the Internet are still operating in a Class environment; therefore, we simply have created a hierarchical routing environment that does not affect the customer environment whatsoever. Let’s start out this discussion by assigning a prefix to the well-known Class addresses. CIDR could operate in a customer environment, but that would require upgrading all routers and hosts to understand CIDR. This is not going to happen. CIDR is primarily used on the Internet routers.

Class A networks have a /8 prefix

Class B networks have a /16 prefix

Class C networks have a /24 prefix

/8? /16? /24? Hopefully, something clicked here! What we have changed to is the network prefix. A network number is basically a network prefix. Nodes on a classless network simply determine the address by finding the prefix value. This value indicates the number of bits, starting from the left, which will be used for the network. The remaining bits are left for host assignment. The prefix can range anywhere from /0 to /32, which allows us to move the network portion of the address anywhere on the 32-bit number.

Imagine then, an address of 198.1.192.0/20. This looks like a Class C address, but the natural mask for a Class C is 24 bits or /24 prefix. This one allows for only 20 bits as the network assignment. But this prefix could be assigned to any address regardless of class. It could be assigned to 15.1.192.0 or 128.1.128.0. The prefix does not care about Class. This is the capability of CIDR. The following section assumes that you can convert binary to decimal and vice versa. If not, please refer to the appendix at the end of this book for an explanation on binary.