Tip

An introduction to IP addressing and subnet masks

Given the complexity of IP addressing and subnet masks, your customers are likely to depend on you to support these services on their IP-based networks. This tip, reposted from SearchNetworking.com's Ask the Expert feature, introduces these concepts

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along with a resource for further information.

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

While it's not possible to fully analyze these topics 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 that's 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 easier 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 allows us to define the range each network will have depending on the class it belongs to.

You might have read or heard that Class C networks can hold up to 255 IP addresses, while Class A networks hold a lot more. While this is correct, it is in reference to the default subnet mask each class has, which determines the amount of networks these classes hold.

By using a 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, 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!

About the author
Chris is the founder and senior editor of Firewall.cx -- a Web site he created to help the IT community benefit from his networking knowledge. Today, Firewall.cx has become a respected Web site with over 450,000 page views per month.

This tip was originally posted on SearchNetworking.com


This was first published in January 2007

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