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IP Addressing

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Class A Addresses

In a Class A network 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

network.node.node.node

For example, in the IP address 49.22.102.70, the 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. As a result,
the maximum number of Class A networks that can be created is 128. Why?
Because each of the seven bit positions can either be a 0 or a 1, thus 2

or 128.

To complicate matters further, the network address of all 0s (0000 0000)

is reserved to designate the default route (see Table 3.3 in the previous sec-
tion). Additionally, the address 127, which is reserved for diagnostics, can't
be used either, which means that you can really only use the numbers 1 to
126 to designate Class A network addresses. This means the actual number
of usable Class A network addresses is 128 minus 2, or 126.

Each Class A address has three bytes (24-bit positions) for the node

address of a machine. This means there are 2

--or 16,777,216--unique

combinations and, therefore, precisely that many possible unique node
addresses for each Class A network. Because addresses with the two patterns
of all 0s and all 1s are reserved, the actual maximum usable number of nodes
for a Class A network is 2

minus 2, which equals 16,777,214. Either way,

that's a huge amount of hosts on a network segment!

Class A Valid Host IDs

Here's an example of how to figure out the valid host IDs in a Class A
network address:

All host bits off is the network address: 10.0.0.0.

All host bits on is the broadcast address: 10.255.255.255.

The valid hosts are the number in between the network address and the

broadcast address: 10.0.0.1 through 10.255.255.254. Notice that 0s and
255s can be valid host IDs. All you need to remember when trying to find
valid host addresses is that the host bits can't all be turned off or all be on at
the same time.

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