I have computer having Broad band connection on it.How the IP address will be assigned to my computer.
Depends on how it is configured. In most cases, the broadband modem gets the IP address from the service (DHCP). If you place a router between it and the computer, it can provide you IP address (same service (DHCP), but the address is likely NAT'ed for privacy). Or you can hardwire the address...
1) IP address of computer is never changed?
If it is hardwired. But if it comes from a service, it has an expiration date.
2)I use Win XP ,If I use get IP address using Command prompt I get a address ,what is this number ? who will assign this address?,is it possible to change this address if so how?
Same question and answer as above.
3) If I get IP address using command prompt differs from address retrieved using some online tool
Not sure of what you are asking.
4) is it possible for a computer to a many ip address??
Each interface can have an IP address. Furthermore, you can assign a virtual IP address too, so you can have a computer respond to many IP addresses on the same interface.
Henry