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 172.16.1.0 and
172.16.2.0. The subnet mask is
255.255.255.0.
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 172.16.1.1 and
172.16.2.1, 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]
