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I didn't forward any ports on my router yet I can still seed in uTorrent. I'm curious as to how this works, because maybe this can give me insight into how I can host Battle.net games without port forwarding.

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quite simply, it connects out to other boxes, but other boxes can't connect to it.

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I was under the impression that every protocol for transferring data has some sort of way of telling the sender that it has received the data. Otherwise, how could things like transfer speed be set? –  soandos Dec 21 '11 at 4:10
    
well, it means that the client without open ports will have to initiate the connection, or do the connection through a system it knows is open. other boxes will not be able to initiate a connection. Since part of the bittorrent protocol does include advertising peers and seeds, things will still work, just slower. –  Journeyman Geek Dec 21 '11 at 5:27
    
But why does it matter who initiates the connection? There still has to be an address for the peer to connect to the seed. –  soandos Dec 21 '11 at 5:31
    
This is completely wrong. What happens is that your PC will talk to your router and dynamically open the ports using uPNP. Connections can absolutely be initiated from other boxes. –  Mundi Apr 25 '12 at 1:38
    
Thats assuming UPNP is switched on. Torrents work perfectly fine without it or port forwarding in many situations –  Journeyman Geek Apr 25 '12 at 8:31
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It is possible your router supports UPnP (Universal Plug and Play). If you have UPnP port mapping configured in uTorrent (check Options -> Preferences -> Connections), then you don't need to forward ports as it is configured automatically.

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Like the Journeyman says, uTorrent can work without port-forwarding.
This should not have any bearing on your Battle.net gaming unless it too supports connectivity without port-forwarding.

Lookup the GameDev.net thread on Host game without port forwarding for some references.

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