Creating subnets

In TCP/IP, the DARPA Internet support includes the concept of the subnet, sometimes called a subnetwork. This is a mechanism that enables several local networks to appear as a single Internet network to off-site hosts. You should consider using subnets in the following instances:

• When you want to hide the local network topology to the outside world. Using subnets requires only a single route to external gateways.

• When you want the ability to administer IP addresses locally. For example, a company may have an engineering subnet, a product marketing subnet, and a sales subnet, each administered by a different administrator who has control of IP addresses in a given range.

• When network bandwidth is limited due to cabling constraints. Setting up subnets, each separated by a gateway host, limits local subnet packets to those that are either destined for or sent from a local host. In this way, the overall network traffic each host on the subnet sees is lessened.

Setting up a subnet consists of:

• determining the appropriate IP addresses for your subnets.

• configuring subnet hosts with the correct IP addresses and netmasks.

• configuring gateway hosts between subnets.

To set up subnet addresses, you must use a segment of the host portion of the IP address to use as the subnet address. For example, consider the following IP addresses and netmasks:

For class A and B networks, you can create subnets by converting the second and third octets, respectively, from host addresses to subnet addresses. Notice how the netmask changes accordingly:

The class A network 16 can now have up to 254 subnets (16.1 - 16.254). The class B network 10.0 can also have up to 254 subnets (10.0.1 - 10.0.254). While the netmask masks the network portion of the address, the broadcast address exposes the network address and hides the host portion. For example, the broadcast address for the subnet 10.0.246, with a netmask of 255.255.255.0, is 10.0.246.255.

For more information on setting netmask and broadcast addresses, see ``Configuration parameters''.

Partitioning a class C address is slightly more complex, as you must take a portion of the fourth octet as the subnetwork. For example, you might partition the first three (high order) bits of the fourth octet to represent the subnetwork, with the last five bits representing the host:

This scheme allows for up to 6 subnets of 30 hosts each, for a total of 180 hosts. The netmask for the hosts on these subnets is 255.255.255.224; 224 is a decimal representation of the binary octet 11100000, which masks the subnet portion of the IP address.

Possible subnets for the class C network 221.138.62.0, with associated broadcast addresses, are:

Subnet	Hosts	Broadcast address
221.138.62.32	.33-.62	221.138.62.63
221.138.62.64	.65-.94	221.138.62.95
221.138.62.96	.97-.126	221.138.62.127
221.138.62.128	.129-.158	221.138.62.159
221.138.62.160	.161-.190	221.138.62.191
221.138.62.192	.193-.222	221.138.62.223

After you determine the new addresses for your hosts, you must configure them with the Network Configuration Manager or by editing /etc/tcp. In addition, you must configure gateways between your subnets: these are hosts with multiple networking cards that serve more than one network.