websites to the same ip address

That's called virtual hosting — Apple calls it "sites" — and that is the default behavior of most any modern web server, and it's what OS X Server provides by default. That is, one IP address, multiple web sites.

Underneath, it's that the web client passes a text string containing the target web site name to the target web server, and the receiving web server looks for that string and uses that to locate and serve up the requested web site, or the default web site, etc., to the requesting web client.