Networking Basics This section introduces the basics of
and there is no security without understanding, so we’re going to briefly discuss bits here. You probably already know that a computer treats all data as zeros and ones, and that a single one or zero is a bit. When a protocol specifies a number of bits, it’s talking about the number as soon by the computer. A 32-bit number has 32 digits, all of which are either one or zero. You were probably introduced to binary math, or base 2, back in elementary school and remembered it just long enough to pass the test. Binary math is simply a different way to work with the same numbers we see every day. In decimal math (or base 10), the math we typically use every day to balance our checkbook or figure out how much over the speed limit we’re driving, digits run from 0 to 9. When you want to go above the highest digit you have, you add a digit on the left and set your current digit to 0. (This is the whole “carry the one” thing you learned many years ago, and now probably do without conscious thought.) Binary math is exactly the same, except that digits run from 0 to 1. When you want to go above the highest digit you have, you add a digit on the left and set your current digit to 0. It’s the same thing, just with fewer digits. Here are the first few decimal numbers converted into binary as an example. Decimal Binary 0 0 1 1 2 10 3 11 4 100 5 101 6 110 7 111 8 1000 When you have a 32-bit number, such as an IP address, you have a string of 32 ones and zeros. Ethernet MAC addresses are 48-bit numbers. Got that? Good. Just to make things difficult, UNIX also uses hexadecimal numbers in some cases (such as MAC addresses and netmasks). Hexadecimal numbers are 4 bits long; each digit goes up to 16. This is accomplished by using the numbers 0 through 9, plus the letters A through F. When you reach the last digit, you reset the current digit to zero and add a digit to the left of the number. For example, to count to sixteen in hexadecimal you go “1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, 10.” Numbers in hexadecimal are usually marked with a leading “0x.” The number 0×11 is a hexadecimal number equal to the decimal number 17, while the number 11 is plain old decimal 11. (If a hex number is not marked by a leading 0x, it’s usually in a place where the output is always in hex numbers.) When you’re working with hexadecimal, decimal, and binary numbers, the simplest thing to do is to break out a scientific calculator. All modern medium-end or better calculators have functions to convert between the three systems. Even the Microsoft Windows calculator has that function. If you want to stick with OpenBSD, you can install /usr/ports/math/hexcalc (see Chapter 13). IP Addresses and Netmasks An IP address is a unique 32-bit number assigned to a particular network node. Some IP addresses are more or less permanent, such as those assigned to servers. Others change as required by the network, such as those used by dial-up clients. Individual machines on a shared network get adjoining IP addresses; we’ll explore what this means a little later. Rather than expressing that 32-bit number as a single number, IP addresses are broken up into four 8-bit numbers. (We’ll see why in a little bit.) These numbers are expressed as decimal numbers. While 192.168.1.1 is the same as the four binary numbers 11000000.10101000.00000001.00000001, or 11000000101010000000000100000001, or even the hexadecimal c0.a8.1.1 or 0×30052000401, the four Page 164
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