HP 200 Series Services And Applications page 111

After you have selected the address class and network number, the rest of
the address bits are allocated to the host field or subdivided into subnet and
host fields. The field lengths chosen for these fields will depend on how the
network is subdivided. For example, a class B address could have 8 bits
assigned to the subnet field and 8 bits to the host field. This would allow up
to 254 subnetworks with up to 254 hosts on each. Or, it could have 9 subnet
number bits and 7 host number bits. This would allow about 500 subnet-
works with up to 126 hosts on each.
Dividing the network into subnetworks helps with the administration of a
large network. When selecting the lengths for the subnetwork and host
fields, consider the total number of subnetworks that will potentially be
needed on the network and the total number of nodes in each subnetwork.
Make sure that the lengths of each field will accommodate these projected
totals and leave room for expansion. (See "Suggestion", following the
discussion of subnet masks below.)
The actual subnet and host number values are assigned by the network
administrator. Neither the subnet or host number can be all 1s or all 0s (255
or 0). These are reserved addresses. You might want to consider using a net-
work number assigned by the Network Information Center at Government
Systems, Inc., even if you are not currently connecting to the Internet. If you
do this now, you will not have to reconfigure your IP network if you connect
to the Internet later.
network
class B
16 bits
network
class B
16 bits
network
class B
24 bits
Internet Protocol Routing Service
IP Addressing Scheme
IP Addresses
host
16 bits
subnet
8 bits
subnet
9 bits
host
8 bits
host
7 bits
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