Glossary

Note: these aren't the definitions you'd find in a book. If that's what you want, then go look in a book. These are they way I would explain the terms to someone. I'd appreciate it if you'd let me know of anything that's misleading or just plain wrong, but I'm not trying to be too strict. Life is tough enough. Sometimes, I may show some sarcasm, though.

Net Mask

A net mask is a 32-bit binary number that consists of a string of 1's followed by a string of 0's. It is sometimes expressed in 'quad-dot' notation, like 255.255.255.0, or simply as the number of consecutive 1's, like /24. It is used to define a sub-net which is separated from all other networks by a router. The netmask indicates which part of the IP address is on the local network, and which must go out through the router. If your ISP assigns you 8 IP addresses, then your netmask will be 255.255.255.248, which means all your IP addresses are the same except for the last 3 bits. Requests for all other IP addresses must go out over the router.

Netmask

A netmask is binary number that consists of a string of 1s followed by a string of 0s. The line between the 1s and 0s is applied to an IP address of a router to define what addresses are on the subnet, below the router, and which ones need to be passed to the higher net above it. Typical netmasks look like 255.255.255.0 (meaning "IP addresses that are the same as mine in the last 8 bits are on the local subnet, and I don't have to pass them on to the network above me. IP addresses that are different than mine in any of the first 24 bits must be passed on." If routers could talk, of course. See subnet, too.

Subnet

A subnet is defined to be part of a larger network by sharing the some number of IP address bits with the larger network. For example, a network might be defined by the upper 16 bits of address to be 216.71 (two octal numbers.) Two subnets under that might be 216.71.130 and 216.71.131. Each of those could have 250 computers on it, each assigned a number between 1 and 254. This completes the "quad-dot" IP address.

The advantage of a subnet is that traffic between two machines on the same subnet doesn't have to go out onto the network above it, reducing traffic on the larger network. The division between shared and local to the subnet is determined by the "netmask". Here's a more detailed writeup.