IP Addresses

For routing to happen, a network design must be based on specific rules. The first rule of TCP/IP is that you must use a valid IP address. An IP address is a series of numbers that represents your computerfor example, 131.108.116.55. Each of the numbers in the IP address is an octet, made up of 8 bits; the whole IP address is 32 bits. If you were to look at 131.108.116.55 in binary, as the computer sees it, you would see 10000011 01101100 01110100 00110111.

IP addresses are typically displayed in decimal format, called dotted-decimal notation. Each host that requires connectivity on a TCP/IP network requires its own unique IP address. A network address in dotted-decimal notation is easier for humans to read than an address in binary form. However, having the address in dotted-decimal notation is not convenient for the computer, which uses binary form. It is sometimes helpful to understand what the computer is doing when it works with network addresses, such as when it is identifying which route may be used for a particular address, determining the binary representation of a subnet mask, or in general, troubleshooting TCP/IP.

The easiest way to convert dotted-decimal notation to binary is to understand the mechanics of the binary system. By knowing that the smallest value an IP address octet can take is 0 and the largest value an IP address octet can take is 255, you can then make a chart such as the one shown in Table 1.1.

Table 1.1. Decimal-to-Binary Conversion Basics

	128(27)	64(26)	32(25)	16(24)	8(23)	4(22)	2(21)	1(20)
255	1	1	1	1	1	1	1	1
0	0	0	0	0	0	0	0	0

Table 1.1 represents the 8 binary digits that make up a single IP address octet. So, if you have a value of decimal 255, its binary equivalent is 11111111. Conversely, if you have a value of decimal 0, its binary equivalent is 00000000. But what about those numbers in the middle? Let's examine the decimal values 127 and 93 in binary notation. Table 1.2 presents these values.

Table 1.2. Converting Decimal Numbers to Binary

	128(27)	64(26)	32(25)	16(24)	8(23)	4(22)	2(21)	1(20)
127	0	1	1	1	1	1	1	1
93	0	1	0	1	1	1	0	1

How is the conversion made? It's simple: 127=64+32+16+8+4+2+1 in decimal notation equals 01111111 in binary notation. In addition, 93=64+16+8+4+1 in decimal notation equals 01011101 in binary notation. The same process can be performed for any other values between 0 and 255.

Exam Alert: Binary Math

Although you're not likely to be tested extensively on binary math on Exam 70-291, it's a good idea to create a chart something like that shown in Table 1.1 when you get into the testing center. This will make it easier for you to quickly perform decimal-to-binary math conversions and also assist you in working with variable length subnet masks (VLSM).

When you know how to convert one octet, you simply apply the process to each remaining octet, and you are in business. With some time and practice, you will be able to quickly convert many common binary values in your head.

Because of the extremely large possible number of IP address2³² (4,294,967,296)IP addresses are organized by class. There are three basic classes: Class A, Class B, and Class C. Table 1.3 defines the range and number of hosts included with each class of network.

Table 1.3. IP Address Classes

Class	Address Range	Number of Network IDs	Number of Host IDs
A	1-126	126	16,777,214
B	128-191	16,384	65,534
C	192-223	2,097,152	254