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Review of IP Addressing
Class A Addresses
In a Class A address, the first byte is assigned to the network address, and the
three remaining bytes are used for the node addresses. The Class A format is
For example, in the IP address, 49 is the network address and
22.102.70 is the node address. Every machine on this particular network
would have the distinctive network address of 49.
Class A network addresses are one byte long, with the first bit of that byte
reserved and the seven remaining bits available for manipulation. Thus, the
maximum number of Class A networks that can be created is 128. Why?
Because each of the seven bit positions can either be 0 or 1, thus 2
or 128.
But to complicate things further, it was also decided that the network
address of all zeros (0000 0000) would be reserved to designate the default
route (see Table 3.1, earlier in this chapter). Thus, the actual number of
usable Class A network addresses is 128 minus 1, or 127. However, the
address 127 is reserved for diagnostics, so that can't be used, which means
that you can use only numbers 1 through 126 to designate Class A networks.
Each Class A address has three bytes (24-bit positions) for the host
address of a machine. Thus, there are 2
--or 16,777,216--unique combi-
nations and, therefore, precisely that many possible unique node addresses
for each Class A network. Because addresses with the two patterns of all
zeros and all ones are reserved, the actual maximum usable number of nodes
for a Class A network is 2
minus 2, which equals 16,777,214.
Here is an example of how to figure out the valid host IDs in a Class A
network. All host bits off is the network address. All host bits on is the broadcast address.
The valid hosts are the numbers in between the network address and the
broadcast address: through Notice that zeros and
255s are valid host IDs. All you need to remember when trying to find valid
host addresses is that the host bits cannot all be turned off or on at the
same time.
When you go out to request a network number from the NIC, don't
expect to be assigned a Class A address. These have all been taken for quite
some time. Big names such as HP and IBM got in the game early enough to
have their own Class A network. However, a check of the IANA records
Copyright ©2001 SYBEX , Inc., Alameda, CA