priller
join:2000-10-20
Gainesville, VA
 ·voip.ms
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·callwithus
| reply to priller
Re: How multiple ATAs can be behind same NAT router?
Packet on the inside | 
Packet after PAT |
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priller
join:2000-10-20
Gainesville, VA
·voip.ms
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| reply to artisticcheese
said by artisticcheese  :I thought UDP are packets are sent and received on the same port number so on router side it shall be receiving also on 5060? That's what you see on the "inside" of your network. But, the port is being changed before it leaves the WAN interface.
The combination of the IP address being translated, as well as the port, is what allows you to have multiple ATA's register to the same VoIP provider and not get confused. |
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artisticcheese
join:2004-11-09
Carrollton, TX | reply to priller
I thought UDP are packets are sent and received on the same port number so on router side it shall be receiving also on 5060? |
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priller
join:2000-10-20
Gainesville, VA
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·Callcentric
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·callwithus
1 edit | reply to DracoFelis
A piece you're missing is that it's really PAT. When the translation is done, your return port number is also changed.
For example, leaving the ATA the UDP SIP packet connects to the host on 5060 and says talk back to me on 5060. Now when the xlate is done, the return port is changed. Here's the connection table for one of my ATA's ....
"UDP PAT from inside:zoom5801/5060 to outside:69.255.118.xxx/1061"
... so the VoIP provider is actually talking back to me on 1061. When the packet passes back through the PAT process, that gets changed back to 5060.
The combination of the IP address and the port number being changed is why you can have multiple ATA's registered to the same provider ... all on what you "think" is 5060. But the router is smarter than we are. 
The translation table should never get confused. If it does, get a new router.
FWIW, I've got 5 VoIP lines living well together. |
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