Sub-netting

Some of you may not have the need, but I will show the rest of you how to subnet a network in this section.

RFC 950 (Request for Comments) defined a standard to divide network classes A, B, C, into smaller pieces, known as subnetting. Believe it or not, subnetting was used to overcome the problem that the Internet faced with the growth problem.

Subnetting allowed a three level hierarchy by introducing the subnet number. It was accomplished by dividing the host number into two parts.

Fig 8.4

As far as the Internet is concerned, the data goes through the router; and the router looks up in its entry to find out if there is a subnet number defined. If yes, the data is sent directly to the host number and doesn't really care where the host is, since it is in the same network prefix. (And by the way the subnet number is not visible outside the internal LAN.)

Fig 8.5

But how exactly is the data handled? When data is requested from an internal LAN to the Internet, it passes through the router encapsulated with an ID number that identifies itself from where the data was requested. The remote server then returns the requested information to the router. The router then acknowledges that host X requested the information. The router verifies in its entry to find out in which subnet host X is located. If the subnet is verified positive, the information then is routed to the destination host “X”.

The internal network can have a very high complexity, but only the router knows it.

Fig 8.6

Observe Fig. 8.6, with a single router = gateway and a single IP address. A system administrator can route several networks to the Internet, without affecting the growth of the Internet. All internal networks can access the Internet as long as the router is active.