893-743-A
F-1
Appendix F IP Addressing
The official description of Internet addresses is RFC 1166, Internet Numbers.
The DDN Network Information Center (NIC) at SRI International in Menlo
Park, California, maintains and distributes the RFC documents. The NIC also
assigns Internet addresses and network numbers. When an organization applies
to the NIC, the NIC assigns a network number or range of addresses that is
appropriate to the number of host devices on the network.
Classes of Internet Addresses
As described in RFC 1166, Internet addresses are 32-bit quantities, divided into
five classes. Classes differ in the number of bits allocated to the network and
host portions of the address. For this discussion, consider a network to be a
collection of computers (hosts) that have the same network field value in their
Internet addresses.
The Class A Internet address format (see Figure F-1) uses the highest eight bits
as the network field and sets the highest priority bit to 0 (zero). The remaining
24 bits form the host field. Only 128 Class A networks can exist, but each Class
A network can have almost 17 million hosts.
Figure F-1. Class A Internet Address
The Class B Internet address format (see Figure F-2) uses the highest 16 bits
as the network field and sets the two highest-order bits to 1,0. The remaining
16 bits form the host field. More than 16,000 Class B networks can exist, and
each Class B network can have up to 65,000 hosts.
Figure F-2. Class B Internet Address
1
7
24
Host
Network
0
1
14
16
Host
0
1
Network
1
