As I’m sure you know, every computer connected to the Internet has an IP address, such as 69.73.181.157 (this site’s IP address). As I’m sure you also know, every computer on the Internet communicates with every other computer via IP address. Most web sites have static (unchanging) IP addresses, while most home users have dynamic addresses (addresses that change every day, week or month, depending on how often your ISP decides to change them). If you still use dial-up, for example, you’ll generally get a different IP address every time you connect to the Internet. Some DSL providers change their customer’s IP address several times a day. Many cable providers don’t change their customer’s IP addresses for months at a time.
So why do web sites have static IP addresses, and why do home users have dynamic ones? Well, web sites need static IP addresses so that people can connect to them. If a website’s IP address changed every day, the Internet’s DNS servers – the computers that convert “yahoo.com” into 216.109.112.135 when you type that address into your address bar – simply couldn’t keep up. The web would be in a constant state of flux, and you wouldn’t be able to connect to your favorite web sites on a regular basis. For an ISP, though, keeping track of which home user has which IP address is a monumental pain. It’s much easier for them to run DHCP servers – computers whose sole job is to assign IP addresses when the home user’s cable\DSL modem asks for one. So rather than have a huge database of account numbers and IP addresses, your ISP simply sets up a server that says “here are our available IP addresses. Whenever a customer requests an IP address, give him one of these”.
It might be helpful to think of IP addresses like phone numbers. A Chinese take-out restaurant needs a static phone number so that people can call in and order food. After all, if their phone number changed every week it’d be hard to order from them, no? But in this example, the restaurant’s customers don’t need static phone numbers to call in an order. They can phone their orders in from their home phone, their work phone, their cell phones, or even a pay phone.