subbnetting

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

- 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 hierarchy to the IP

addressing structure. Instead of the classful two-level hierarchy, subnetting supports a

three-level hierarchy. Figure 6 illustrates the basic idea of subnetting which is to divide

the standard classful host-number field into two parts - the subnet-number and the hostnumber

on that subnet.

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

subnet the destination 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 individual 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

complexity into the private network without affecting the size of the Internet's routing

tables.

Subnetting overcame the registered number issue by assigning each organization one (or

at most a few) network number(s) from the IPv4 address space. The organization was

then free to assign a distinct subnetwork number for each of its internal networks. This

allows the organization to deploy additional subnets without needing to obtain a new

network number from the Internet.

In Figure 7, a site with several logical networks uses subnet addressing to cover them

with a single /16 (Class B) network address. The router accepts all traffic from the

Internet addressed to network 130.5.0.0, and forwards traffic to the interior subnetworks

based on the third octet of the classful address. The deployment of subnetting within the

private network provides several benefits:

- The size of the global Internet routing table does not grow because the site

administrator does not need to obtain additional address space and the routing

advertisements for all of the subnets are combined into a single routing table entry.

- The local administrator has the flexibility to deploy additional subnets without

obtaining a new network number from the Internet.

- Route flapping (i.e., the rapid changing of routes) within the private network does

not affect the Internet routing table since Internet routers do not know about the

reachability of the individual subnets - they just know about the reachability of the

parent network number.