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

Chapter 77
Example One: An ISP Address Assignment

Let’s look at another example, this time using a better example of address assignment: the Internet Service Provider. The ISP block is 200.24.0.0/16. Hmmmmm. Looks a little strange. This is a Class C address, but there is a 0 in the third octet and the prefix (subnet mask) is only 16-bits wide. The natural mask is 24 bits (255.255.255.0). This is known as supernetting, and will be shown in the next pages, so bear with me here.

A customer of the ISP needs three subnets, each supporting 60 hosts. Remember, we assign the mask contiguous starting from the left. Since subnets are divided evenly (due to the binary nature of the address), we cannot have three subnets without dividing the address to provide for four subnets. The address assigned to the customer is 200.24.255.0/24. Therefore:

1.  How many bits are needed in the subnet mask to support three subnets?

2.  2ⁿ2 = 4, therefore 2 bits are required in the subnet mask. This leaves one left over but masks must be contiguous.

Example One: An ISP Address Assignment

3.  This leaves 6 bits left for host assignment. 2ⁿ6 leaves 62 (2ⁿ6 =64 and we subtract 2 because we cannot have all 0s or all 1s in the host portion of the address) address assignments for hosts, and therefore we can use this single network address assignment for our company.

This should make you a little nervous. There are only two hosts per subnet left for expansion and there is only one subnet left. The ISP should make very sure that this company will not grow anytime soon.