shows that several corporations were handed Class A addresses back in 1995
and that Stanford University's Class A was revoked in July 2000. The
records also indicate that the IANA has control of many Class A addresses,
ones that have not been allocated to regional ISPs. A company can also buy
another company to get a Class A network ID. For example, Compaq got the
16 network by acquiring Digital.
Class B Addresses
In a Class B address, the first two bytes are assigned to the network address,
and the remaining two bytes are used for host addresses. The format is
For example, in the IP address 172.16.30.56, the network address is 172.16,
and the host address is 30.56.
With a network address being two bytes of eight bits each, there would be
65,536 unique combinations. But the Internet designers decided that all
Class B addresses should start with the binary digits 1 and 0. This leaves 14
bit positions to manipulate; therefore, there are 16,384 unique Class B
A Class B address uses two bytes for node addresses. This is 216 minus the
two reserved patterns (all zeros and all ones), for a total of 65,534 possible
node addresses for each Class B network.
Here is an example of how to find the valid hosts in a Class B network:
172.16.0.0 All host bits turned off is the network address.
172.16.255.255 All host bits turned on is the broadcast address.
The valid hosts would be the numbers in between the network address and
the broadcast address: 172.16.0.1 through 172.16.255.254.
Just as we saw with Class A addresses, all Class B addresses have also been
assigned. Many universities, which were connected to the Internet in the
early '90s, in addition to many big-name organizations such as Microsoft,
Cisco, Xerox, Novell, and Sun Microsystems, have all of these addresses
consumed. However, they are available under the right circumstances.
Class C Addresses
The first three bytes of a Class C address are dedicated to the network por-
tion of the address, with only one measly byte remaining for the host
address. The format is
Copyright ©2001 SYBEX , Inc., Alameda, CA