How are IP addresses and subnet masks calculated?

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IP addressing and subnet masks are one of the most popular topics in the networking community simply because they can be very confusing and require some time to sink in.

While it's not possible to fully analyze the topic in one page, I'll give you a bit of information to start off with and some guidance to help you move deeper into the subject.

An IP address is an address that helps us uniquely identify a network device or host.

When configuring a computer with an IP address we define the logical network it is part of. A logical network is not something we can touch or see, but a term used to describe the way certain things are perceived by the computer or network device.

The IP address given to the computer tells it which network it belongs to, and how it will identify itself to the rest of the computers that are part of the same network. The subnet mask sets the network's boundaries.

IP addresses exist in both public networks (the Internet) and private ones (LANs) and since there are millions of them, it was decided to put them in specific classes to help organize the IP addressing structure and make it more easy to work with.

Today all IP addresses can be categorized into five different classes, each class having a specific range:

IP Classes	Default Subnet Mask
Class A: 1.0.0.0 to 127.255.255.255	255.0.0.0
Class B: 128.0.0.0 to 191.255.255.255	255.255.0.0
Class C: 192.0.0.0 to 223.255.255.255	255.255.255.0
Class D: 224.0.0.0 to 239.255.255.255
Class E: 240.0.0.0 to 255.255.255.255

Out of the five classes, the first three, A, B and C are used on the Internet by its users in order to communicate, while the rest, D and E, are reserved for other reasons. In most cases, you will always be working with Classes A to C.

Each Class was also given a certain subnet mask, called the "default subnet mask." The default subnet mask allowed us to define the range each network would have depending on the class it belonged to.

You might have read or hear people saying that Class C networks can hold up to 255 IP addresses, while Class A networks hold a lot more. While this is correct, they are really referring to the default subnet mask each class has, that determines the amount of networks these classes hold.

By using a different subnet mask, other than the default, we are able to further split the networks into smaller ones to suit our needs.

Instead of continuing on the analysis of subnets, I'll refer you to my Web site (www.firewall.cx/) which covers the topic in the best possible way using easy to understand diagrams to help you "see" what happens during the break down.

If you find the information overwhelming, it might comfort you to know that it took me some time to fully understand it back in the days I was introduced to the topic!