Subnetting

In 1985, RFC 950 defined a standard procedure to support the subnet 

ting, or division, of a single Class A, B, or C network number into

smaller pieces. Subnetting was introduced to overcome some of the

problems that parts of the Internet were beginning to experience with

the classful two level addressing hierarchy, such as:

  Internet routing tables were beginning to grow.

  Local administrators had to request another network number from the

Internet before a new network could be installed at their site. 

Both of these problems were attacked by adding another level of hierar 

chy to the IP addressing structure. Instead of the classful two level hier 

archy, subnetting supports a three level hierarchy. Figure 7 illustrates

the basic idea of subnetting, which is to divide the standard classful

host number field into two parts the subnet number and the host num 

ber on that subnet. 

F I G U R E   7 .   S u b n e t   A d d re s s   H i e r a rc h y

Subnetting attacked the expanding routing table problem by ensuring

that the subnet structure of a network is never visible outside of the

organization's private network. The route from the Internet to any sub 

net of a given IP address is the same, no matter which subnet the desti 

nation host is on. This is because all subnets of a given network number

use the same network prefix but different subnet numbers. The routers

within the private organization need to differentiate between the indi 

vidual subnets, but as far as the Internet routers are concerned, all of

the subnets in the organization are collected into a single routing table

entry. This allows the local administrator to introduce arbitrary com 

plexity into the private network without affecting the size of the Inter 

net's routing tables. 

Subnetting overcame the registered number issue by assigning each

organization one (or at most a few) network numbers from the IPv4

address space. The organization was then free to assign a distinct sub 

network number for each of its internal networks. This allowed the

organization to deploy additional subnets without obtaining a new net 

work number from the Internet. 

U N D E R S TA N D I N G   I P   A D D R E S S I N G

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