Documentation for WinProxy

Subnets

You may be asking: "So what the heck is a subnet mask, and why is it important?" If so, read on!

Have you ever noticed that IP addresses are often specified in pairs (IP address and subnet mask)? The subnet mask tells you-more importantly, it tells the tcp/ip stack in your computer-two things about the IP address: (1) which part of the address is designated as the network identifier; and, (2) which part designates individual connections on that network.

For the casual user, the most important thing to take from this is that the subnet mask must be the same on every network connection within that subnet. Thus, when setting set up your local network, use the same subnet mask throughout. You needn't use the one we recommend for new users-255.255.255.0-but that's a simple and common way to set up a small to moderate local network, and its hard to go wrong with it.

To see how a subnet mask works, we change the usual form of IP addresses to the "real" form, the 32-bit binary number for which it stands:

The portion of the IP address that corresponds to ones section of the subnet mask is the network identifier. The portion with the zeroes is for individual addresses. The subnet mask must have all ones to the left, and all zeroes to the right. In the example above, the left-most 24 positions of the IP address specify the network address, accounting for the other notation you sometimes see: 90.0.0.0/24. This latter notation gives the same exact information as the IP address/ subnet mask pair.

The eight positions on the right are for individual addresses on the 90.0.0 network. The lowest number (00000000) and the highest number (11111111) are reserved for special uses. The lowest number-90.0.0.0-is called the "network address." It's used in routing tables and other places; it's also commonly used as the name to designate the entire subnet. The highest number, 90.0.0.255, is called the "broadcast address."

This leaves you 00000001 through 11111110 for your addresses, i.e., 1 through 254. (Or, impress your friends by calculating it this way: eight binary digits with two exceptions is 28-2, or 256-2 = 254 total addresses available). The first address available for your use in the 90.0.0.0 subnet is 90.0.0.1. This is the number that we usually recommend for the WinProxy computer. There's nothing particularly special about the "1"-the unique identifier- it's just easy to remember and fast to write down.

On a simple local network, the network address portion of the IP address and the subnet mask must be the same for each computer. In our example here, all local computer addresses would start with "90.0.0" and each and every computer would have a subnet mask of 255.255.255.0. Only the individual address portion of the IP address will be different, and that portion must be unique for each connection.

Subnet masks don't have to be on the byte boundaries. As an example, let's do this with 90.0.0.0/29:

As you can see, you have the right-hand three bits for individual addresses. 23-2 gives you six usable addresses on each network. Thus, 90.0.0.0 is the network address, 90.0.0.7 is the broadcast address, and you can use 90.0.0.1 through 90.0.0.6 for individual addresses. You don't lose all that other space, though.