The longer answer to why one web site can have two (or more) addresses involves learning a little about IP numbers. What are they? IP numbers, or Internet Protocol numbers, are assigned to computers on the Internet. Again, when we speak of Internet addresses, think street addresses—web servers have street addresses, in this case IP numbers, that allow them to be found by other computers searching for that address. A web server administrator can assign several addresses to one computer.
The problem is that an IP address is a number, usually in four parts, identifying a computer on the internet. This is not a problem for computers as they work well with numbers, but people sometimes have trouble remembering all of those IP numbers. We can remember street addresses and phone numbers, usually, but what if the address were 35.8.69.129? That's not so easy to remember. So there is a service called a Domain Name System (DNS) that helps us, the people, remember the computer's addresses.
Network administrators can assign a DNS to a specific IP addresses if that name is not already taken and being used by another computer. In other words, if I have a server on the Internet with the IP number 11.2.33.444, I can have a DNS assigned to make the address http://www.myserver.com point to the IP number 11.2.33.444.
The address of my computer-it's IP number-has not changed, but the method we use to describe that address has changed. After the DNS is registered, a web user could put either11.2.33.444 into their address field and find my server, orthey could enter http://www.myserver.com. But, and this is very important, only one DNS can be used to point to one address. If http://www.myserver.com is already taken and being used by someone else, I would have to come up with another address. I would not change my IP number, but the DNS pointer for that IP would have to be different. Any DNS can point to any IP, but each DNS must be unique. So, that is why web users can type either http://www.matrix.msu.edu or http://matrix.msu.edu and get the same page. Both addresses are assigned to the number 35.8.69.129. Both DNS addresses are unique (one has "www" and the other does not), but both are assigned to the same IP number. Follow the links below to see this in action:
MATRIX webserver: