decimal numbers are easiest to work with. [3]
decimal numbers are easiest to work with. [3] If your company is hooking up to the Internet, your ISP will issue you a block of IP addresses. Frequently this is a small block, say, 16 or 32 IP addresses. If your system is colocated on a server farm, you might only get a few IP addresses. It all depends upon your needs. The size of your IP block determines your netmask or, the size of your netmask determines how many IP addresses you have. If you’ve done networking for any length of time, you’ve seen the netmask 255.255.255.0. You might even know that the wrong netmask will keep your system from working. In today’s world, that simple netmask is becoming less and less common. To understand why this is, you need to understand something about the history of IP addressing. Many years ago, IP addresses were issued in blocks of three sizes: class A, class B, and class C. (There were also a few chunks of class D and class E space, but those really aren’t relevant to the discussion.) This terminology has been obsolete for quite some time, but we’ll use it as a starting point. Class A was very simple: The first of the four numbers in an IP address were fixed. The InterNIC might issue you a class A like “10.0.0.0.” You could assign any of the last three numbers in any manner you liked, but all your IP addresses began with 10. For example, you could delegate 10.1.0.0 through 10.1.1.255 to your data center, 10.1.2.0 through 10.1.7.255 to your Detroit office, and so on. Only very large companies, such as Ford and Xerox, as well as influential academic computing institutions such as MIT, received class A blocks. In a class B block, the first two of the four numbers in the IP address were fixed. Your class B block would look something like 192.168.0.0. Every IP address you used internally began with the first two numbers 192.168, but you could assign the last two numbers as you wanted. Many midsized companies got class B blocks. Similarly, a class C block had the first three numbers fixed. This was the standard for small companies. The ISP would issue a block like 209.69.178.0 and let you assign the last number as you wanted. This scheme wasted a lot of IP numbers. Many small companies don’t need 256 IP addresses. Many medium-sized companies need more than 256, but fewer than the 65,000 in a class B block. And almost nobody needs the full 16 million addresses in a class A block. Still, those were the choices. Before the Internet boomed, they were good enough. Remember, back in the 1980s the thought that private individuals would hook up to the Net from home, for entertainment, was laughable. Today, IP addresses are issued by prefix length, commonly called a slash. You will see IP blocks such as 192.168.1.128/25. While this looks confusing, it’s merely a way of using classes with much greater granularity. You know that each number in an IP address is 8 bits long. By using a class, what you’re saying is that a certain number of bits are “fixed” you cannot change them on your network. A class A address has 8 fixed bits, a class B has 16, and a class C has 24. This isn’t a class in binary math, so I won’t make you draw it out and do the conversion. But think about an IP address as a string of binary numbers. On your network you can change the bits on the far right, but not the ones on the far left. There’s no reason that the boundary between the two must be on one of those convenient 8-bit lines that separate the decimal versions of the numbers. A prefix length is simply the number of fixed bits you are stuck with. A /25 means that you have 25 fixed bits, or one more fixed bit than what used to be called a class C. You can play with 7 bits. In the following sample, your fixed bits are all ones, and the bits you can change are zeros. 11111111.11111111.11111111.10000000 It’s very simple if you think in binary. You won’t have to work with this every day, but if you don’t understand the underlying binary concepts, the decimal conversion looks like total gibberish. With practice, you’ll learn to recognize some bits of decimal gibberish as legitimate binary conversions. So, that’s the theory. What does this mean in practice? Page 165
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