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

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Chapter 47
Reasons for Subnetting

As network numbers were assigned, many sites were implementing routing on their local sites. This had many benefits. You could have many networks at your site (using RFC 791), but the problem was that you had to be given multiple network addresses (Class A, Class B, or Class C) to accomplish this. This started to fill up the ARPAnet routing tables and created other problems as well.

Many networks that accessed the Internet were creating their own home–grown subnetted environments, and many were beginning to be implemented. Before all networks ceased communicating because of incompatibilities, RFC 950 was released, defining a standard method for subnetting an IP address. A network mask that covers simply the network portion of the address is known as the natural mask (no portion of the address is subnetted).

The slide shows a subnetted network topology connected to the Internet. It is assigned a Class B address and uses an 8–bit subnet mask. The Internet knows of the IP address 130.1.0.0. It does not know the subnets involved. This allows the Internet address (routing) tables to remain smaller.

Subnet masks are used in routers and network stations.

Reasons for Subnetting

•  Most IP address assignments were not used very efficiently.
•  Having millions of hosts for Class A and 254 hosts for Class was not working very well
•  Many sites were requesting multiple network numbers due to variable amounts of networks at their sites.
•  Many networks were implementing proprietary subnets.
•  RFC 950 defined the adopted subnet method.


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