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're 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'd 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, and about half of the total available IP addresses have already been assigned. However, new techniques for working with IP addresses have helped to alleviate this problem, and a new standard for 128-bit IP addresses (known as IPv6) is on the verge of winning acceptance.

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 consists of two parts:

• The network ID (or network address): Identifies the network on which a host computer can be found.

• The host ID (or host address): Identifies a specific device on the network indicated by the network ID.

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. The original IP specification uses the address classes system to determine which part of the IP address is the network ID and which part is the host ID. A newer system, known as classless IP addresses, is rapidly taking over the address classes system. You come to grips with both systems later in this chapter.

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, known as an octet, is represented by its decimal equivalent. For example, consider the following binary IP address:

 11000000101010001000100000011100 

The dotted-decimal equivalent to this address is

 192.168.136.28 

Here, 192 represents the first eight bits (11000000); 168, the second set of eight bits (10101000); 136, the third set of eight bits (10001000); and 28, the last set of eight bits (00011100). This is the format in which you usually see IP addresses represented.