I mentioned earlier that certain network numbers are "special", and that they would be bad choices. You may also realize (or need to know) that if the machines you are assigning numbers to are connected directly to the Internet, without going through a firewall or router that is translating or otherwise proxying network addresses, the choice of numbers isn't up to you at all. That's because the IP addresses used on the Internet are specifically asigned to the people who own them: you can't just grab any arbitrary number out of a hat and use it. You have to get your own assigned IP addressees.
However, most small business networks either are not connected at all, or will be connected through a NAT (Network Address Translation) router or through a proxy firewall. It's a little too early to discuss either of these subjects, so let's just move along with you happily assigning your own IP addresses.
First, you can't use 127 as the first number. This is because the address 127.0.0.1 is reserved as a "loopback" address; an address that always refers to the machine at hand. In other words, every machine on your network can telnet to 127.0.0.1 and they will always end up at themselves, and not anywhere else. What's the point? Actually, it's useful. Consider the case of a client-server application where you want to run the client on the same machine as the server. Or consider a programmer who wants to test a network program, but doesn't want to go out on the real network just yet.
You also may have had support people ask you to "ping 127.0.0.1" to test your TCP/IP configuration. What's being tested there is strictly TCP/IP, with the network card not in the loop, and no interference or confusion from the rest of the network. It's strictly local, strictly a loopback. It needs to be there: don't ever delete this address from /etc/hosts.
The next addresses you shouldn't use are anything beginning with 224 or above. These are the Class D and Class E addresses (that's old terminolgy, but people still use it, so will I), reserved for special use. One use being made of them now is for Multicast addresses, where one computer sends data but many other computers receive it.
Just about anything else is available to you, following the rules that none of the four numbers can be 0 (really, some of them could be zero, but it's less confusing right now if we pretend that they cannot) and none of them can be 255. Keep in mind that the rules about 127 and 224 and up only apply to the first number of the four: you shouldn't use 126.96.36.199, but 10.225.1.8 is fine.
By the way, these four numbers are usually referred to as "octets", simply because they hold numbers that can be expressed in 8 bits. If it makes your head hurt to talk about bits, this might be a good time to bail out, because we're going to be getting into that pretty heavily from this point on. On the other hand, I'm going to try to make it painless, so you might consider hanging in there.
Don't panic, but we're going to talk bits!
An eight bit number can be anything from 0 to 255. This is because of the value of the bits. The 0 bit (computer geeks number things starting with 0, not to annoy you, but because it's easier when working with low-level computer registers) is worth 1 if set (and 0 if not), the 1 bit is worth 2 (again, if set, and zero if it isn't), the 2 bit is 4, and so on. Usually that's illustrated something like this:
More Articles by Tony Lawrence © 2011-03-01 Tony Lawrence
It all sounds good from the pulpit,but come Monday morning all the sinners are back to business as usual writing crappy code. (Tony Lawrence)