Skype is a peer-to-peer (P2P) application, meaning that users connect to one another directly and not through a central server for communication. Skype initially uses Internet-based servers to authenticate users when they log in and to track their status, but when a "chat" or instant message, "voice call" or "file transfer" is initiated, the parties involved in the communication do so in a P2P direct connection. If one or both of the users are behind a typical corporate Network Address Translation (NAT) firewall, the communication can be relayed through a Supernode because a direct P2P can't be established behind a NAT. In the case of a file transfer, you will see a message indicating your transfer is being relayed.
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