[Erwin]

An TCP connection is uniquely identified by a {local IP, remote IP, local port, remote port} tuple.

So if you are on 192.168.1.1 and want to connect to a specific port on 192.168.1.2 there aren't enough free port numbers to get 1 million connections. Thus the extending of the "ephemeral port range" (the local port number the kernel is allowed to assign) and addition of more local IPs.

[gilgoomesh]

Except there's no assumption here that the 1 million connections are between just two computers. Clients are spread over 50 different EC2 instances (which each have a unique address). The host does not need more ports in this scenario and the clients are using 20,000 ports (possible without altering port allocation).

[Erwin]

This was a reply to tmartiro's thread, where he says "You can also open 1 million connections using one linux box" and describes how you could do it a single instance rather than 50 separate ones; adlpz asked "what would be the point of that".

[adlpz]

Well, of course, you can only have 65535 established connections sending data concurrently at a given point in time. But what I meant is, that is not the bottleneck at all, but the software behind handling all these requests, so tmartiro's comment was either pointless or sarcastic.

Note: This scenario is only valid for two computers talking to each other. As gilgoomesh said, if you have multiple clients you have virtually unlimited valid connection tuples (src addr, dst addr, src prt, dst prt).