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

Chapter 86
A Look at the Addresses of an ISP

An ISP has been assigned this block from the InterNIC:

209.16.0.0/16.

At first glance, this address looks like a Class C, but the prefix does not match a Class C. It is a Class B prefix. Again, this is known as supernetting and shows that CIDR does not care about Classes. With a prefix of /16, this would represent 256 Class C addresses. However, in CIDR, the ISP is free to choose any method of segmenting this address and handing it out to its customers. The ISP also knows that IANA and the InterNIC do not just hand out lots of addresses; therefore, the ISP is very careful about carving up the addresses.

The ISP pulls off a portion of the address space using a /20 prefix: 209.16.16.0/20. This represents a small portion of the addresses, or 16 Class C addresses. The ISP leaves the upper 4 bits of the address reserved for future use. 209.16.16.0/20 is the address space that we will work with. Based on some surveys with its customers, the ISP cuts the address into two pieces yielding 209.16.16.0/21 and 209.16.24.0/21. (For those not familiar with binary, shifting right 1 bit divides the number by 2. Shifting left 1 bit multiplies the number by 2). 209.16.16.0/21 (eight Class Cs) is assigned to a single customer. The other half of the address, 209.16.24.0, is cut up again into three pieces:

209.16.24.0/22 representing _ of the address (four Class Cs)

209.16.28/23 representing 1/8 of the address (two Class Cs)

209.16.30.0/23 representing 1/8 of the address (two Class Cs)