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@alex So the destination port is used to identify the service, as you know 80 for http 22 for ssh etc.

But multiple clients can connect to those servers at the same time, even multiple connections from the same client. So how does it tell those connections apart?

So the client picks a random source port to initate the connection. From 1.1.1.1:12345 to 2.2.2.2:80 for example.

This mapping is the basis of the TCP connection, so the server at 2.2.2.2:80 knows to send back to 1.1.1.1:12345 for this particular connection.