Assuming there's only one ip address bound to the machine in question then the ip address that IIS will be using is the ip address of the machine.
If you're planning on forwarding port 80 from a single public ip address to multiple internal ip addresses then you're going to need to use something like ARR to do it.
IIS by default runs on the same ip address as the computer unless you add multiple ip addresses to the computer and bind IIS to one of those additional ip addresses.
If the computers are getting those ip addresses via DHCP then there's no guarantee that the computers will always get the same ip address. You can assign a static ip address to each computer if you need them to always have the same ip address