IP addresses are divided into two types: static and dynamic. Static IP addresses do not change, and dynamic addresses do.

Static IP addresses are used when you need to make sure that the same service is available at the same IP address for a long time.
Dynamic IP addresses are used when you don't, or rarely, need to initiate a connection to your computer from somewhere else. For example almost all the connections you make using a home computer are outbound; you are initiating a connection to a server somewhere else. You probably never attempt to access your computer from a remote location, so you don't need to know or care what your IP address is. The fact that it might change doesn't break anything.

Dynamic IP addresses are the result of the days when most computers were often only temporarily connected to the internet. When a computer using dynamic IP addressing connects it asks for and is assigned an IP address right then and there. When it disconnects, that IP address is released and can be assigned to any other computer to come along after and ask for one. That way a large number of IP addresses did not have to be reserved for a bunch of computers that connected only briefly. Instead the IP addresses could be reused and assigned to other machines as needed.

Dynamic addressing is also somewhat more convenient. If you had a static IP address you'd almost certainly know it, because it requires that you manually configure all the information about your internet connection. Using a dynamic IP your computer can automatically configure itself.

So, if your computer is connected continuously, but uses a dynamic IP address, when might it change and why?

There's no hard and fast rule. When a computer is assigned a dynamic IP it is also told how long it's allowed to use it. Before that time is up, it's required to ask for an IP again. Most of the time the networking equipment will re-assign it the same address. That's nice, but not required.

The most common example of why something might change is network maintenance. Your ISP may elect to alter or repair their network in some way. Reassigning your machine a new IP address the next time it asks is an easy way for your ISP to begin routing your connection through different equipment - with you never noticing. Heck, even rebooting a router, be it yours or your ISPs, is quite likely to cause your IP address to change eventually.

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If you need to be able to initiate connections to your machine from remote locations, you need to know the IP address. In general there are three options:

•Have your machine do something that makes its IP address visible somewhere on the internet that you can find it somehow. This is harder than it sounds. You need your IP address to get to your machine, but you need to get to your machine to get your IP address. You're stuck!

•Get a domain name, and use a dynamic DNS service to map it to your machine's current IP address. This typically requires a small piece of software running on your machine that notices the IP address change and informs the DNS service. The downside is that when your IP address changes there can sometimes be significant delays before the DNS servers are updated world-wide.

•Get a static IP address. The most reliable option, but it's typically an extra-cost option from most ISPs and will require some additional configuration of your either your machine, or your router, or both.

But again, for the majority of average internet users, dynamic IP addressing is the norm, and the fact that the IP address might change is nothing to worry about.

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