Hmm, Networking 101 ;)

Ok... assuming this is IPv4 were talking about (it is), an IP address is a 32bit number . It is included in IPv4 packets and is used by systems to route the data.

In the Internet, it works as so:
1. ICANN (Internet Corporation for Assigned Names and Numbers) delegates 'blocks' of IP addresses to RIRs (Regional Internet Registries)
2. RIRs delegate 'blocks' of IP addresses to LIRs (Local Internet Registries - typically your ISP)
3. LIRs delegate IP addresses to users.

There are proboably lots of rules governing RIRs and LIRs but I don't do internet politics.

Anyway, most end users will be given a 'dynamic' IP. In other words, your IP will be change and be within one of the ISP's blocks. It may change whenever you reconnect, hourly, daily, monthly, daily and whenever you reconnect, whenever you reconnect if you've been disconnected for more than n, when a full moon occurs at the same time as a solar storm is occuring, etc.

As for IP's being easily spoofed: yes. As I said, it's just a bit of data in a packet... BUT, it's not easy to get it out on the Internet anymore.

Most ISPs will block packets containing an IP address they consider based on differing criteria invalid. Even if they don't, you'll only be able to spoof the IP address within the ISP as whatever the ISP is connected to will most likely block packets not coming from within the ISP's network, etc. But it gets worse - or better (it mostly depends on which way you enjoyed rsh ;) - even if you manage to spoof your IP address, will you get the data back? Uh, no.. the person holding that IP address will or - as the case may be - won't. But you won't get it. And it's a lot harder to pretend you are (and there are good - or bad (it mostly depends on which way you enjoyed rsh ;) - reasons for doing so) now, too.

But, as mentioned, there are plenty of proxies aronud. And IP addresses change in some cases. And people get cracked. And people quit an ISP and somone gets their IP. So, very bad idea to use it even as just a possibility. Hell, it's profiling! It's discriminating against people who run Tor servers, for instance...