Increasingly, computers are positioned behind firewalls to protect systems from internet threats. Ideally, the firewall function will be performed by a router, which also translates the PC’s local network address to the public IP address (Network Address Translation, or NAT). This means an attacker cannot directly adress the PC from the outside – connections have to be established from the inside.
This is of course a problem when two computers behind NAT firewalls require to talk directly to each other – if, for example, their users want to call each other using Voice over IP (VoIP). The dilemma is clear – whichever party calls the other, the recipient’s firewall will decline the apparent attack and will simply discard the data packets. The telephone call doesn’t happen. Or at least that’s what a network administrator would expect.
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