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 (several millions). Medium-sized Class B networks have two-byte network numbers, with the first byte in the range 128 to 191; they support several thousands of host, but are almost exhausted. Class C networks are the smallest and the most commonly available; they have three-byte network numbers, with the first byte in the range 192-223. Class D (multicast, 126.96.36.199 to 188.8.131.52) and E (research, 240.0.0.0 to 255.255.255.255) also have three-byte network numbers.
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 (this is answered by the #loopbackHost class method).
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 specifies all four bytes of the address individually; a.b.c interprets as a 2-byte quantity, which is useful for specifying host addresses in a Class B network with network address number a.b; a.b intrprets the last part of the address as a 3-byte quantity, which is useful for specifying host addresses in a Class A network with network address number a.
If only one part is given, this corresponds directly to the host address number.