somebody told me that one computer has only one ip address?

I think thats a wrong notion .as many have already told here,depending on the network your system can have more than one ip.also if you have two different operating systems suppose windows and linux.you can set two different ips in these operating systems.

you need not have only one IP address. it depends on the number of network connections you have, and also each ones setting. You have one IP address for each network you conect to. For LAN (Local Area Network) you'll have a unique IP address in the network. This can be changed to obtain a IP address automatically but you'll never get connected this way. Best thing is to have you IP address as 192.168.100.1 for the domain computer(main) and increase the last number by 1 or 2 for every other computer. But no two computers can have the same. For the internet it works in the same way. Broadband and DSL Cable will require to have a constant IP address, depending on your ISP (Internet Service Provider). For Dial-up modems it is recommended to obtain an IP address automatically. But if you prefer to use the same IP address everytime (60.1.xx.xxx or 61.1.xx.xx) you will have a tough time trying to figure out the DNS that goes along with it. Hope I helped.

Well that might not always be true. Depends on what type of IP adress you reffer to. One computer can have more than one IP. If the computer has two networks (usually LAN and internet) than it has two IP adresses for both networks. The LAN IP always starts with 192.168 and the internet IP adress depends on the IP that your ISP gave you and may differ very much. There are adresses like 60.xxx.xxx.xxx and adresses like 220.xxx.xxx.xxx