Map ip address to localhost

The only thing is that you're not saying "all addresses should have access" -- that's done in your firewall(s) and/or the server software and/or other security layers like tcpwrappers.

0.0.0.0, in this context, means "all IP addresses on the local machine" (in fact probably, "all IPv4 addresses on the local machine"). So, if your webserver machine has two IP addresses, 192.168.1.1 and 10.1.2.1, and you allow a webserver daemon like apache to listen on 0.0.0.0, it will be reachable at both of those IP addresses. But only to what can contact those IP addresses and the web port(s).

Note that, in a different context (routing) 0.0.0.0 usually means the default route (the route to "the rest of" the internet, aside from routes in your local network etc.).

Is there a simple way to display the local IP address on screen via a client-side script?

No, at least I don't know of any cross-platform way to do this in Javascript. It's unlikely there is any, since this would be a considerable security hole (any site you visit could retrieve your networking configuration, which might be considered private information and could help in attacks). It's possible using IE and ActiveX, but I wouldn't recommend it (not cross-platform, don't know if it even works in recent IEs).

You could do it from a Java applet (if it is signed), as you have full system access (might also be possible from Flash).

That said:

Maybe you can explain why you believe you need this. Why do users have to "register" to you? Why do you even need their IP address?

Why don't you just give out IP addresses via DHCP?

If you explain your problem / goal in more detail, maybe there's a better way.