Why dosen't Tor always know the external IP address?

Question

The "getinfo address" command documentation says:

"address" -- the best guess at our external IP address. If we
  have no guess, return a 551 error. (Added in 0.1.2.2-alpha)

Why doesn't Tor always know exactly what the exit node's IP address is?

Answer 1

There are multiple possibilities:

• The server has no external IP. Nothing is stopping you from starting the Tor relay software on a server that isn't connected to the Internet.
• The server is connected to the internet, but sits behind a NAT and the mechanism to detect the external IP (e.g. a third-party website) is down.
• There is an address that looks like an external IP, but the server is not reachable at that address. I'm actually not sure how Tor handles that case - it might be that this is not checked here, but somewhere else down the line.