Caveats are that I don't really do Windows, I don't know AISmon and due to the combination of a recent disc calamity and being at the end of my data plan until the new "month" starts in a few days, I don't currently have the OpenCPN
If using OpenCPN
4.0 and AISmon is sending data to 127.0.0.1, udp port 10110 on the data connections screen
Connection type "Network"
Priority (unless you have other AIS
sources, doesn't matter)
Checksum: Yes if AISmon correctly outputs this
Check "Receive input on this port" (should be default) and don't check "Output on this port" (unchecked should be default).
IIRC (see "don't have source", above :-) whatever you put for address, OpenCPN will *receive* on all addresses for all interfaces it listens on so best to make that explicit with the wildcard "0.0.0.0")
With the "I don't know much about windows" caveat applied (so "generally speaking"), you should be able to send to a loopback interface if an external interface is down. If Windows can't do that I'd find it "surprising". If you can send when the external interface is up I'd be questioning where AISmon is really sending the traffic. In many unix-y OSes it's possible for a privileged application to override the routing table and send via a particular interface but (a) that's network-fu people don't generally do and (b) there's no *need* for it here. Remember, OpenCPN will receive on any address it listens for (i.e. all its interface addresses and all broadcast addresses) so just because you put "127.0.0.1" into the box doesn't mean that's what the data is being sent to when it successfully receives.
I'd normally attack the problem with wireshark: see what's being sent on the loopback interface and see what's being sent on the external interface when tethered, but that's a bit too involved for many people. Got a pointer to the documentation
for AISmon? A bit of googling didn't turn it up for me