Is there a way to ask the Airport Extreme what it's external IP is via terminal?
Is your Airport the 'last hop' before the ISP? Straight into the modem, no other router between you &amp; them?

If so, then asking Google 'what's my IP address?' will show it. 

That address is the 'border' between you &amp; the outside world &amp; in fact it kind of is partly at your Airport &amp; partly at the next hop, your ISP, insomuch as it's your last reference point, &amp; their first.
You'd need a hardware engineer to clarify that last point - the details are beyond me, I'm afraid.