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Networks 101

© March 1998 Tony Lawrence

Most of this article is applicable to any Unix or Linux, but some examples of "route" commands are not.

There are a few fundamental ideas about TCP/IP networks that people seem to get confused about. This article might help.

See also Routing and Advanced TCP .

First, let's pretend that you are setting up a few machines, but they will not be connected to the Internet. They are all yours, under your control. In this situation, you can use any IP addresses you want; the addresses just have to meet certain rules:

IP addresses look like this:

Each of the numbers between the dots must be between 1 and 254 (except sometimes some of the numbers can be zero's, but for now we won't use those). For reasons we'll go into later, a good choice would be to use some number beginning with 10, but you can choose something else if you insist. The very worst choice you could make would be 127 (for the very first number only; it's OK elsewhere), so stay away from that one for now (it's special).

You'll also need a "subnet mask". Your choice for that depends on how many machines will be on this network. Your choices would be:

Machines                Mask
up to 254     
up to 64,516  
up to 16,387,064

It is possible that you may have been assigned a number or range of numbers and a mask by someone else. If so, just go with the flow for now and pretend that isn't true. If it is true, the number(s) you have may be a little strange because they are "supernetted", or assigned out of a specific block of numbers. This will be covered in another article (November 1998, I hope), but for now just accept that you have something different and this "basics" article doesn't cover that.

If you don't have an assigned number, then you should use one in the following ranges:

Nowadays, those ranges would usually be referred to with their CIDR notation:

Every machine needs a unique number. So, if I were setting up the very first machine, I might assign it, and then the next, and so on. What I can't do is decide that machine number 4 will be instead of And, if I used the subnet mask of, I can't assign the next machine either.

The reason for that has to do with that mysterious subnet mask, and this is where it gets confusing. Really, it's simple: it's just one of those silly things that seem confusing at first.

Think of the "255"'s as markers that determine how many of the numbers in your network addresses cannot be changed (that is, if they were changed, the address would be on a different subnet). If your network mask is and you started numbering machines as , then you cannot change the 10.1.1 part in any address. So all of these numbers are good:

but all of these are "bad" (that is, they are on a different subnet):

Why does it matter if they are on a different subnet? Because you can't talk (telnet, ftp, ping) to machines on a different subnet without a route. There are good reasons why you might actually want this (such as having too many machines for one subnet, or for security, or for performance reasons), but we'll talk about those in another article.

If you had used as your mask, then only the 10.1 part would be frozen, and you could use:

but not:

Remember the was just one choice we could have made. Instead we might have chosen as the address for our first machine. If our mask again were , then the 172.16.35 would be the "fixed" part of our addresses.

Why would you choose one network mask over another? Good question. If you think about it, one answer might be because your choice affects how many hosts (machines) you can have in the one subnet. If the mask is, then the first three octets (geek talk for the first three numbers; "octet" because they are 8 bit numbers) are fixed; you can't change them without changing the subnet. That would leave only 0 to 255 for hosts, and you can't (I haven't told you why yet, so just accept it for now) use 0 or 255 here, so that's only 253 machines, which might not be enough even if you aren't General Motors Corp. So you might need to use to get more hosts.

That "fixed" part is actually the "network" address. I said earlier that you can't change any of those fixed numbers. If you do, you are creating another network, and if machines on one network (say 10.1.1) need to talk to machines having a different network address (all beginning with 172.16.76, for example), they need to do so through a "router", which can be a machine with two (or more) network cards in it, or a specialized piece of equipment. Whatever it is, the router will always have addresses on every network it is connected to.

So, a router might have on one port, and on another. Visualize a bunch of machines to the left of this box all having addresses beginning with 10.1.1, and another bunch of machines to the right of it all with addresses beginning 192.168.15.

If a machine on the left wants to talk to a machine on the right, it cannot unless it has a route to that other machine. In many cases, those are the only other machines that the machine on the left can talk to, so the route becomes the default route.

In every case, when setting a route, the route must be to a machine on the same network (with the same "fixed" numbers) as the machine you are setting it on. So, if you are setting a route on machine, the address on the router must be the 10.1.1 address. It cannot be the 192.168.15 address or any other address on the router. This is because ordinary machines (machines that are not routers) can ONLY talk to machines on their very own network. So, if you were working on machine 10.1.30 in this imaginary network, and you wanted to set a route to the 192.168.15 network, you might say:

route add

or even:

route add default

What you'd be saying in the second case is "send anything that isn't on my network to, and (I hope) that machine will get it to where it needs to go".

To make that route permanent, you need to do more. For SCO, see Routing. For NT, a /P adds the route to the registry. Linux has a slightly different format for the command; you need a "gw" keyword after default, and to make it permanent you'd usually use "linuxconf".

Changing an ip address after you've assigned it can be more work than you might think. For SCO, see Jeff Liebermann's SCO Page See also:

Advanced TCP



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