The addressing scheme that is used today in the Internet is based on version 4 of the Internet Protocol (IPv4), usually referred to simply as IP. This section discusses the following:
• Basic IP addressing
• Basic IP subnetting
• Variable-length subnet mask (VLSM)
Basic IP Addressing
An IP address is a unique 4-octet (32-bit) value expressed in dotted-decimal (or dotted-quad) notation of the form W.X.Y.Z, where periods (dots) are used to separate each of the 4 octets of the address (for example, 10.0.0.1). The 32-bit address field consists of two parts: a network or link number (which represents the network portion of the address) and a host number (which identifies a host on the network segment). The network and host boundaries were traditionally defined based on the class of the IP address, with five defined classes (three of which are used for unicast addressing): A, B, C, D,
and E. Table illustrates the different classes of address space and their functions.
Notice that only Class A, B, and C addresses are used for unicast. Class D addresses are used for multicast, and Class E address space is reserved. Several addresses within these classes are reserved for special use. Table2 lists some of these addresses.
This class-based addressing scheme is often referred to as the classful model. The different classes lend themselves to different network configurations, depending on the desired ratio of networks to hosts. The full implications of the different classes will become more apparent as this chapter proceeds. The next few sections focus on the basic definitions of each class.
Class A Addressing
Class A networks are represented by a 0 in the leftmost bit position of the address. The first octet (bits 0 to 7) of the address, beginning from the leftmost bit, represents the network number, and the remaining 3 octets (bits 8 to 31) represent a host number on that network. An example of a Class A network is 124.0.0.1, where 124.0.0.0 represents the network number and the host number is 1. The outcome of this representation, illustrated in Figure 1, is 128(2^7) Class A network numbers. However, because 0.0.0.0 is not a valid network number, only 127(2^7–1) Class A addresses are possible.
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