Depends on the context of your qn, but:
1. dynamic IP.
Used by ISPs who have eg 10,000 users, but only eg 3,000 logged in at any time. IP addrs are a limited resource.
There is a protocol DHCP that can be used to manage this and it tells the client what the nameservers are and a bunch of other stuff, optionally.
Big company's also use this because there's a large turnover in PCs and its easier to automate management via DHCP than try to keep a master list and manually set IP every time.
2. virtual IP
normally, each NIC has only one IP addr, but you can add more to it using 'virtual' method eg eth0 becomes eth0:0, eth0:1 etc.
see also Apache virtual hosts: http://www.linuxtopia.org/online_boo...ettingupvhosts