Basics on subnet masks and IP addresses

I've seen several different Web site addresses in literature with different subnet masks -- i.e. x.x.x.x:8080 or x.x.x.x/27.

I'm assuming that both formats are just different ways of writing the mask although I realize I could be wrong.
I assume your new to the exciting world of networking and it seems there are a few minor points that might not be clear, so let's have a quick overview on a few things.

As you would know, every host on the Internet or a network is identified by their IP Address, which is the x.x.x.x part you mentioned in your question.

When we contact a host, we need to let it know the reason we are contacting it. A simple example is a Web server, which you contact in order to download a Web page. The method used to let the server know what you need is by using what we call a 'port' number.

A server might have 5 different services running: I.E. www, ftp, telnet, dns and smtp. The server distinguishes between these services by letting them run on different ports. The port number identifies the specific service on that server, for example, the www service usually runs on port 80.

When you sent a request to download a Web page on that server, your computer sends the request to the server's IP address, I.E. and to port number 80. A simple and efficient way to type this is the following:

From this interpretation ,you can start to see that the x.x.x.x:8080 format you mentioned, does not indicate any subnet mask, but an IP address, followed by a port number of 8080.

Port number 8080 is usually used for proxies. If you have a Web proxy in your network, all clients of that network send their request to the proxy, and the proxy will fetch the information on their behalf:

PC----PROXY----Internet Server

In the above diagram, the PC sends a request to the proxy, which in its turn sends the request to the Internet server. The Internet server responds back to the proxy with the data, and the proxy then forwards the information to your PC!

This way of communication is used to provide a more efficient use of the available bandwidth and also to make sure the PC's are not directly connected to any hosts on the Internet, so it's also a way of increasing security.

On another note, when you type a URL in your browser's URL address bar: I.E. www.searchnetworking.com, the http:// is automatically added, and the computer knows to send the request to searchnetworking's.com Web server on port 80. If your Web browser is configured to use a proxy, then it will forward the request to the proxy's IP address and to the port you have specified.

Coming to the second part of your question, the x.x.x.x/27, you are correct! The /27 is simply a way of describing how many subnet bits are used for the particular network. In this specific example, it indicates 27 subnet bits which translates to a subnet mask of

The /27 format is usually called CIDR – Classless InterDomain Routing, and is a big subject in and of itself. You need a firm understanding on subnetting before you can tackle CIDR. My suggestion is to visit www.Firewall.cx and read the subnetting topic (Networking/Protocols/Internet Protocol-Subnetting).

Once your comfortable with the concept, tackle the CIDR topic which is available on the site.

This was first published in August 2003

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