| Tuesday May 31st 2016

How Skype and company get around firewalls


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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