NAT
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... OR3: The OP must be able to communicate through uncooperative
firewalls, NATs, and when the computer is disconnected from the
Internet.
...
... Discussion: Since the evil-doer may have control of a local
firewall or NAT, the OP must be able to communicate with the OP
server, even when the firewall or NAT ...
... NAT, the OP must be able to communicate with the OP
server, even when the firewall or NAT has been configured to
block all unused ports. Also, since the evil-doer might try to
...