LITMIR.BIZ

      FIGURE 3-1: The Windows Calculator in Programmer mode.

Introducing IP Addresses

      An IP address is a number that uniquely identifies every host on an IP network. IP addresses operate at the network layer of the TCP/IP protocol stack, so they are independent of lower-level data link layer MAC addresses, such as Ethernet MAC addresses.

IP addresses are 32-bit binary numbers, which means that theoretically, a maximum of something in the neighborhood of 4 billion unique host addresses can exist throughout the Internet. You’d think that would be enough, but TCP/IP places certain restrictions on how IP addresses are allocated. These restrictions severely limit the total number of usable IP addresses. Many experts predict that we will run out of IP addresses soon. However, new techniques for working with IP addresses have helped to alleviate this problem, and a standard for 128-bit IP addresses has been adopted, though it still is not yet in widespread use.

      Networks and hosts

      IP stands for Internet Protocol, and its primary purpose is to enable communications between networks. As a result, a 32-bit IP address actually consists of two parts:

      Most of the complexity of working with IP addresses has to do with figuring out which part of the complete 32-bit IP address is the network ID and which part is the host ID, as described in the following sections.

      The dotted-decimal dance

      IP addresses are usually represented in a format known as dotted-decimal notation. In dotted-decimal notation, each group of eight bits — an octet — is represented by its decimal equivalent. For example, consider the following binary IP address:

       11000000101010001000100000011100

      To convert this value to dotted-decimal notation, first divide it into four octets, as follows:

       11000000 10101000 10001000 00011100

      Then, convert each of the octets to its decimal equivalent:

       11000000 10101000 10001000 00011100192 168 136 28

      Then, use periods to separate the four decimal numbers, like this:

       192.168.136.28

      This is the format in which you’ll usually see IP addresses represented.

Figure 3-2 shows how the 32 bits of an IP address are broken down into four octets of eight bits each. As you can see, the four octets of an IP address are often referred to as w, x, y, and z.