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

Chapter 57
Assigning More Than One Address to an Interface

Have a network with 275 hosts but you were assigned all Class C addresses? What can you do here? TCP/IP fully supports the ability to assign more than one subnet or network number to the same segment. Actually the router vendors implemented this as an ability of TCP/IP. This means that one network may employ more than one network number on the same physical cable plant. In order to accomplish this, a router must be used. Network stations continue to believe they are communicating with a remote network station, but the router is simply providing the address translation. The packet goes in one port and then right back out the same port. The two nodes actually reside on the same network segment. A router will take the steps necessary to allow network stations to converse on the network. Implementations are different, so the amount of network numbers that may be assigned to the same cable plant varies.

For example, as shown in the slide, multiple Class C network numbers may be assigned to the same cable plant. Class C addresses allow only for 254 host IDs per network number. This is a rather low number, and some sites will have more than 254 network stations attached to a cable plant. This means that multiple stations on the same cable plant may have different network addresses. A router must be used to translate between two stations that are located on the same cable plant with different network addresses. This is called multinetting an interface.

Assigning More Than One Address to an Interface