IP Addressing
IP addressing is the heart of the TCP/IP-based internetwork. The process of routing IP packets is possible because of this logical addressing scheme.
An IP address is a logical 32-bit binary number that identifies a system on an internetwork. An IP address comprises two parts—the network portion and the host portion. The network portion of an IP address tells the host what logical network it is located on. The host portion identifies that particular host.
IP Address Format
Because humans tend to have trouble remembering and evaluating binary numbers, IP addresses are expressed in dotted decimal notation. A 32-bit binary IP address is written out in four octets, each of which contains eight bits. Each bit position in an octet represents a value (one of 128, 64, 32, 16, 8, 4, 2, 1); the sum of these values, when totaled, represents the octet’s decimal value (see Figure 1.9).

Figure 1-9. Dotted decimal example
IP Address Classes
Initially, when IP was developed, the IP address space was divided into distinct IP address classes to determine where the network portion stops and the host portion begins. The value of the first octet and its highest order (leftmost) bits determine the class. There are five IP address classes, three of which (A, B, and C) are available for commercial use (see Figure 1.10). Class D is reserved for IP multicasting. Multicasting allows multiple computers ...
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