5.5. Creating Subnets

To operate several Ethernets (or other networks, once a driver is available), you have to split your network into subnets. Note that subnetting is required only if you have more than one broadcast network—point-to-point links don't count. For instance, if you have one Ethernet, and one or more SLIP links to the outside world, you don't need to subnet your network. This is explained in more detail in Chapter 7.

To accommodate the two Ethernets, the Brewery's network manager decides to use 8 bits of the host part as additional subnet bits. This leaves another 8 bits for the host part, allowing for 254 hosts on each of the subnets. She then assigns subnet number 1 to the brewery, and gives the winery number 2. Their respective network addresses are thus and The subnet mask is

vlager, which is the gateway between the two networks, is assigned a host number of 1 on both of them, which gives it the IP addresses and, respectively.

Note that in this example we are using a class B network to keep things simple, but a class C network would be more realistic. With the new networking code, subnetting is not limited to byte boundaries, so even a class C network may be split into several subnets. For instance, you could use two bits of the host part for the netmask, giving you 4 possible subnets with 64 hosts on each.[1]



The first number on each subnet is the subnetwork address, and the last number on each subnet is reserved as the broadcast address, so it's actually 62 hosts per subnet.