Private IP Addresses

In the same way that you can’t just randomly choose the numbers to use for an IP address, you also need to be careful with the addresses you ultimately use. IP addresses used on the public Internet are assigned to companies from organizations like RIPE (the European IP address registry), or from an ISP. Although using a range assigned to a company might work on your home network, it can also impact your ability to connect to certain Internet resources. It’s for that reason that “private” IP addresses exist.

Private IP addresses were designated as a solution to the public Internet quickly running out of available addresses. As the Internet has grown, the number of available unique IP addresses has quickly dwindled. In order to satisfy the need for more IP addresses, certain ranges were designated as private, or available for anyone (including home or business networks) to use. These addresses are not valid on the public Internet, so they do not impact TCP/IP communication outside of a network. For example, you could be using the same private IP addresses on your network as your neighbor is, and the whole Internet would still function in peace and harmony.

This begs the question – if private IP addresses aren’t valid on the Internet, how can the computers on your home network access Internet resources? The answer is found in something called Network Address Translation (NAT). When a computer using a private IP address wants to access the Internet, that private address must be “translated” to a public address that is valid on the Internet. On your home network, one system (such as a dedicated router or one of your PCs) will still need at least one public address that will be shared amongst your internal computers. On systems like Windows 98 or XP, this functionality is provided by a service known as Internet Connection Sharing, or ICS. More on ICS and other NAT techniques will follow later in the series.

For now, the most important thing for you to remember is that you should always use private IP addresses on your internal network. These are first and foremost more secure, and will help you to avoid problems later. The private IP address ranges available to anyone who wants to use them are: to to to

In general, most home users tend to stick with addresses that start with 192.168, and you should as well to keep things simple. For example, if you start all of your IP addresses with 192.168.1.X, you can support up to 254 IP addresses on your home network, which should be more than you would ever need.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.