12 Networking: A Beginner’s Guide Y ou don’t need to have a Ph.D. in computer science to be an effective networking person, but you do need to understand some rudiments of the subject. This chapter discusses basic computer terminology and knowledge that you should possess to make the information in the rest of the book more useful and understandable. If you’ve been working with computers for a while, and especially if you have training or experience as a computer programmer, you might not need to read this chapter in detail. However, it is a good idea to at least skim it, to make sure that you understand these subjects thoroughly. Bits, Nibbles, and Bytes Most people know that computers, at their most fundamental level, work entirely using only 1s and 0s for numbers. Each of these numbers (whether it is a 0 or 1) is called a bit, which is short for binary digit. String eight bits together, and you have a byte; string about 1,000 bits together, and you have a kilobit; or you can string about 1,000 bytes together for a kilobyte. A rarely used unit is composed of four bits strung together, called a nibble. Remember this for when you play Jeopardy! Understanding Binary Numbers Before you learn about binary numbers, it’s useful to recall a few things about the numbering system that people use on a daily basis. This is called the decimal numbering system or, alternatively, the base-10 numbering system. The decimal numbering system is built using ten different symbols, each of which represents a quantity from zero to nine. Therefore, ten possible digits can be used: 0 through 9. (The base-10 numbering system gets its name from the fact that only ten digits are possible in the system.) An important part of any numbering system is the use of positions in which the numerical symbols can be placed. Each position confers a different quantity to the number being represented in that position. Therefore, the number 10 in the decimal system represents the quantity ten. There is a 1 in the tens position and a 0 in the ones position. This can also be represented as (1×10) + (0×1). Now consider the number 541. This number uses the hundreds position as well as the tens and ones positions. It can be represented as (5×100) + (4×10) + (1×1). In English, you could state this number as five hundred plus forty plus one. Every written number has a least-significant digit and a most-significant digit. The least-significant digit is the one farthest to the right, and the most-significant digit is the one farthest to the left. For binary numbers, people also talk about the least- and most-significant bits, but it’s the same idea. So far, this section has simply reviewed basic number knowledge that you learned in grade school. What grade school probably didn’t cover is the fact that basing a numbering system on ten is completely arbitrary; there is no mathematical reason to favor a base-10 system over any other. You can create numbering systems for any base 13 Chapter 2: Laying the Foundation you like. You can have a base-3 numbering system, a base-11 numbering system, and so on. Humans have come to favor the base-10 system, probably because we have ten fingers and thus tend to think in tens. Computers, on the other hand, have only two digits with which they can work—1 and 0—so they need to use a different numbering system. The natural numbering system for a computer to use would therefore be the base-2 numbering system, and, in fact, that’s what they do use. This system is called the binary numbering system. Computers use only 1s and 0s at their most basic level because they understand only two states: on and off. In the binary numbering system, a 1 represents on, and a 0 represents off. Recall that in the decimal numbering system, the position of each number is important. It is the same in the binary numbering system, except that each position doesn’t correspond to powers of 10, but instead to powers of 2. Here are the values of the lowest eight positions used in the binary numbering system: 128 64 32 16 8 4 2 1 So, suppose that you encounter the following binary number: 1010110 1 You would follow the same steps that you use to understand a decimal numbering system number. In this example, the binary number represents 128 + 32 + 8 + 4 + 1, or 173 in the decimal system. You can also write (or calculate) this number as follows: (128 × 1) + (64 × 0) + (32 × 1) + (16 × 0) + (8 × 1) + (4 × 1) + (2 × 0) + (1 × 1) So, two main things separate the decimal numbering system from the binary numbering system: N The binary system uses only 1s and 0s to represent every value. N The value of numerals in the different positions varies. You might be wondering how you can tell whether you’re reading a binary number or a decimal number. For instance, if you’re reading a book about computers and you see the number 10101, how do you know whether it’s supposed to represent ten thousand one hundred and one or twenty-one? There are several ways that you can tell: N Usually, binary numbers are shown with at least eight positions (a full byte), even if the leading digits are 0s. N If you’re looking at a bunch of numbers and see only 1s and 0s, it’s a pretty good bet that they are binary numbers. N Binary numbers don’t use the decimal point to represent fractional values, so 10100.01 should be assumed to be a decimal system number. 14 Networking: A Beginner’s Guide N Decimal numbers should use commas as you were taught in school. So, the number 10,100 should be read as ten thousand one hundred, whereas the number 10100 should be read as the binary number for the quantity twenty. N Sometimes people put the letter b at the end of a binary number, although this convention isn’t widely followed. Put all these things together, plus a little common sense, and you’ll usually have no doubt whether you’re reading a binary or decimal value. Other Important Numbering Systems Two other important numbering systems that you encounter in the world of networking are octal and hexadecimal. Hexadecimal is far more prevalent than octal, but you should understand both. The octal number system is also called the base-8 numbering system. In this scheme, each position in a number can hold only the numerals 0 to 7. The number 010 in the octal numbering system corresponds to 8 in the decimal numbering system. Octal numbers can be indicated with a leading zero, a leading percent symbol (%), or a trailing capital letter O. The hexadecimal numbering system is fairly common in networking, and is often used to represent network addresses, memory addresses, and the like. The hexadecimal system (also called the base-16 numbering system) can use 16 different symbols in each of its positions. Since we have written numerals for only 0 to 9, the hexadecimal system uses the letters A through F to represent the extra symbols. How to Quickly Convert Hexadecimal, Decimal, Octal, and Binary Numbers The Calculator application that comes with all versions of Windows allows you to convert values quickly between hexadecimal, decimal, octal, and binary. With the calculator open, place it into Scientific mode (open the View menu and choose Scientific). This mode reveals a lot of advanced features in the calculator. In the upper-left area of the calculator, you can now see four option buttons labeled Hex, Dec, Oct, and Bin. These correspond to the hexadecimal, decimal, octal, and binary numbering systems. Just choose which system you want to use to enter a number, and then click any of the other options to convert the number instantly. For instance, suppose that you click the Bin option button and enter the number 110100100110111010. If you then click the Dec button, the calculator reveals that the number you just entered is 215,482 in the decimal system. If you click the Hex button, you find that the binary number that you entered is 349BA in the hexadecimal numbering system. And if you click the Oct button, you discover that the number is 644672 in the octal numbering system. You can also go in the other direction: Click the Dec button, enter some number, and then click the other option buttons to see how the number looks in those other numbering systems. 15 Chapter 2: Laying the Foundation Hexadecimal numbers are usually preceded with a leading zero followed by the letter x, and then the hexadecimal number. The letter x can be either lowercase or uppercase, so both 0x11AB and 0X11AB are correct. Hexadecimal numbers may also be shown with a trailing letter h, which can be lowercase or uppercase. Rarely, they may be preceded with the dollar sign ($), as in $11AB. Often, you can easily recognize hexadecimal numbers simply by the fact that they include some letters (A to F). For hexadecimal numbers, A equals 10 in the decimal system, B equals 11, C equals 12, D equals 13, E equals 14, and F equals 15. You can determine the decimal value for a hexadecimal value manually using the same method as shown earlier in this chapter for decimal and binary numbers. The hexadecimal position values for the first four digits are as follows: 4096 256 16 1 So, the number 0x11AB can be converted to decimal with the formula (1 × 4096) + (1 × 256) + (10 × 16) + (11 × 1), or 4,523 in decimal. Basic Terminology to Describe Networking Speeds The business of networking is almost entirely about moving data from one point to another. Accordingly, one of the most important things that you need to understand about any network connection is how much data it can carry. Broadly, this capacity is called bandwidth, which is measured by the amount of data that a connection can carry in a given period of time. The most basic measurement of bandwidth is bits per second, abbreviated as bps. Bandwidth is how many bits the connection can carry within a second. More commonly used are various multiples of this measurement, including thousands of bits per second (Kbps), millions of bits per second (Mbps), or billions of bits per second (Gbps). TIP Remember that bits per second is not bytes per second. To arrive at the bytes per second when you know the bits per second (approximately), divide the bps number by 8. In this book, bits per second units are written with a lowercase letter b and bytes per second units with an uppercase B. For example, 56 Kbps is 56 thousand bits per second, and 56 KBps is 56 thousand bytes per second. A closely related measurement that you will also see bandied about is hertz, which is the number of cycles being carried per second. Hertz is abbreviated as Hz. Just as with bps, it is the multiples of hertz that are talked about the most, including thousands of hertz (KHz, or kilohertz) and millions of mertz (MHz, or megahertz). For example, a microprocessor running at 100 MHz is running at 100 million cycles per second. The electricity in the United States runs at 60 Hz; in Europe, the speed is 50 Hz. 16 Networking: A Beginner’s Guide Hertz and bits per second are essentially the same and are sometimes intermixed. For example, thin Ethernet cable is said to run at 10 MHz and also to carry 10 Mbps of bandwidth. Chapter Summary This book would double in size if I tried to explain every networking term every time it was used. To keep it at a reasonable length (and readable), I assume that you understand the basic concepts presented in this chapter, as well as the information found in the glossary near the end of the book. Most people leave glossaries unread until they come across a term they don’t know. In this case, I recommend that you spend a few minutes reviewing this book’s glossary before you read the following chapters, to make sure that you are familiar with the terms that are used. Node, host, broadband, baseband, workstation, client, and server are some examples of terms that the rest of the book assumes that you understand. The glossary defines these terms and many others. In the next chapter, you learn about the basic types of networks, as well as an important conceptual model of networking that you will frequently encounter when working with networks: the Open Systems Interconnection (OSI) model. The OSI model is used in virtually every aspect of networking, and it provides a framework for how networks operate. . B equals 11, C equals 12, D equals 13, E equals 14, and F equals 15. You can determine the decimal value for a hexadecimal value manually using the same method as shown earlier in this chapter. things that you need to understand about any network connection is how much data it can carry. Broadly, this capacity is called bandwidth, which is measured by the amount of data that a connection. uppercase, so both 0x11AB and 0X11AB are correct. Hexadecimal numbers may also be shown with a trailing letter h, which can be lowercase or uppercase. Rarely, they may be preceded with the dollar