Beginners DNS question

I've set up ISPConfig 2 on a SuSE 11.3 box. I want to add a couple of new sites on the server. What I don't understand is that I only have one IP address assigned by my ISP. Do I have to have BOTH 'A' records for the domains I want to host pointing to THAT SINGLE IP address and then let ISPConfig sort out who is being addressed or do I need to have my ISP assign a new IP address for every site I'm hosting?

I'm sure that this has been addressed some where in the 100+ pages of the forum but I couldn't find it.

Thanks

Jim

Note: As a beginner I'm using 2 instead of 3 until documentation is available!