Punchthrough appears to not be happening between 2 computers on same network

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hi All. Question regarding the punch through. I'm running an app on two computers on the same network (test 1), and same network but 1 behind a software vpn (test 2), and on 2 computers in different networks (test 3)

Only in test 3 is there a punch through attempt and success. The others don't even show any punches being sent or received in the logs...what's going on?

for reference - in test 1, computer 1 can ping computer 2's local ip...in test 2, it can't