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NullOrEmpty
Asked: 2012-11-27 03:48:55 +0800 CST

NAT: If two hosts initiate a connection to the same IP:PORT, with the same source PORT, how does the router handle it?

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I was just thinking and that idea came to my mind.

Let say I have hosts A and B inside my local area network. I have a Router, and a host C in the internet. I know that when initiating the connection, the source port is chosen randomly, but what happens if two hosts chose the same when initiating to the same host:port?

A:10123 -> C:80
B:10123 -> C:80

How would the router know which packets belongs to each one?

Cheers.

networking

3 Answers

  1. Best Answer

    Just because hosts A and B chose port 10123 doesn't mean that is the port that the router uses. The router connects to host C from a source port of it's own choosing. So the connection looks more like this:

                                      router
                          +--------------------+
A:(src:10123, dest:80) -> | A:10123 -> C:43775 | -> C:80
B:(src:10123, dest:80) -> | B:10123 -> C:43776 | -> C:80
                          +--------------------+
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  2. I am with Laurentiu the Router will send the Packet based on the IP Address from A or B. The Response Packet would look like this:

    Source IP: C
Source Port: 80
Destination IP: A (or B)
Destination Port 10123

    the Router would Route the Packet to the right Host.

    If the Router is configured with PAT the Router would assign the NAT/PAT IP and new random Source Ports for the outgoing Packets.

    • 1

  3. Router has routing table with ip address . It remember the particular ip address to find which host made request and route that response packet to that particular host.

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