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Internet Host Addresses

An Internet host address is a number containing four bytes of data. These are divided into two parts, a network number and a local network address number within that network. The network number consists of the first one, two or three bytes; the rest of the bytes are the local address.

Network numbers are registered with the Network Information Center (NIC), and are divided into three classes--A, B, and C. The local network address numbers of individual machines are registered with the administrator of the particular network.

Class A networks have single-byte numbers in the range 0 to 127. There are only a small number of Class A networks, but they can each support a very large number of hosts. Medium-sized Class B networks have two-byte network numbers, with the first byte in the range 128 to 191. Class C networks are the smallest; they have three-byte network numbers, with the first byte in the range 192-255. Thus, the first 1, 2, or 3 bytes of an Internet address specifies a network. The remaining bytes of the Internet address specify the address within that network.

The Class A network 0 is reserved for broadcast to all networks. In addition, the host number 0 within each network is reserved for broadcast to all hosts in that network.

The Class A network 127 is reserved for loopback; you can always use the Internet address `127.0.0.1' to refer to the host machine.

Since a single machine can be a member of multiple networks, it can have multiple Internet host addresses. However, there is never supposed to be more than one machine with the same host address.

There are four forms of the standard numbers-and-dots notation for Internet addresses:

a.b.c.d
This specifies all four bytes of the address individually.

a.b.c
The last part of the address, c, is interpreted as a 2-byte quantity. This is useful for specifying host addresses in a Class B network with network address number a.b.

a.b
The last part of the address, c, is interpreted as a 3-byte quantity. This is useful for specifying host addresses in a Class A network with network address number a.

a
If only one part is given, this corresponds directly to the host address number.

Within each part of the address, the usual C conventions for specifying the radix apply. In other words, a leading `0x' or `0X' implies hexadecimal radix; a leading `0' implies octal; and otherwise decimal radix is assumed.

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