Networking :: Addressing Internally And Externally With A /30?

It's more just straight networking rather than Linux related - possibly a rather large gap in my knowledge.Been asked to set up a Linux firewall / router for a friend, and he wants an external address on his PC and his firewall. He's got a /30... ISP seem to think this is normal. My understanding was different router interfaces should be in different subnets when it comes to addressing. eg:

WAN 1.1.1.1LAN 192.168.1.1/24 (then PC 192.168.1.2)This is how I'd set up an enterprise router, and from memory how I had to regurgitate networking for my Cisco exams. But they would have been large devices within a BGP environment, how does this compare to home use? Can you have interfaces on the same router within in the same subnet? I'm looking for the real world answer - not my CCNA answer or the config addressing scheme I just blindly followed...Is this right or can different interfaces be in the same subnet?If they can be in different subnets How would you do that with an eternal 1.1.1.1/30? egWAN 1.1.1.1LAN (what IP?) then PC 1.1.1.2Would you actually just set the router up as a /29 and then use the additional IP addresses for the LAN NIC on the router? (yes I'm aware you couldn't route to where it had actually been allocated)

Then with that how do I route the 1.1.1.1/30? Which interface do I point it at? Even with a /29 I'd still have a routing issue right?Or are the ISP expecting you to do something with NAT / port forwarding. (and yes I could do this to get it working - but I want to understand it better) I'm asking the question here, because rather than just wanting a magic file to get things working, I'd actually rather understand the principles rather than just press a button and have it work.