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IP Addresses

An IP address consists of two parts: a network ID, which specifies the network on which a host resides, and a host ID, which identifies the host within that network.

IPv4 addresses

An IPv4 address consists of 32 bits (Figure 58-5). When expressed in human-readable form, these addresses are normally written in dotted-decimal notation, with the 4 bytes of the address being written as decimal numbers separated by dots, as in 204.152.189.116.

An IPv4 network address and corresponding network mask
Figure 58-5. An IPv4 network address and corresponding network mask

When an organization applies for a range of IPv4 addresses for its hosts, it receives a 32-bit network address and a corresponding 32-bit network mask. In binary form, this mask consists of a sequence of 1s in the leftmost bits, followed by a sequence of 0s to fill out the remainder of the mask. The 1s indicate which part of the address contains the assigned network ID, while the 0s indicate which part of the address is available to the organization to assign as unique host IDs on its network. The size of the network ID part of the mask is determined when the address is assigned. Since the network ID component always occupies the leftmost part of the mask, the following notation is sufficient to specify ....


  

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