With a maximum of 64k TCP connections per single server IP, you need 77 different IP on the server side.
This is a fact.
Having run production services that had over 250,000 sockets connecting to a single server port, I'm calling "nope" on that.
Are you thinking of the ephemeral port limit? That's on the client side; not the server side. Each TCP socket pair is a four-tuple of [server IP, server port, client IP, client port]; the uniqueness comes from the client IP/port part in the server case.
The real problem with such a setup is that you're not left with a whole lot of bandwidth per connection, even if you ignore things like packet loss and retransmits mucking up the connections. Most VPS servers have a 1gbps connection, with 5 million clients that leaves 200 bytes per second of concurrent bandwidth for TCP signaling and data to flow through. You'll need a ridiculous network card for a single server to deal with such a load, in the terabits per second range.
Cloudflare has some interesting blog posts on this topic:
- https://blog.cloudflare.com/how-we-built-spectrum/
- https://blog.cloudflare.com/how-to-stop-running-out-of-ephem...
If you suppose just one open server port, you’ll probably need 77 client ips to do this test to get unique socket pairs.
But it’s a client problem, not a server one.
Clients can connect to the server on the same server port, so connection limit is more like 64k*2 for every Client IP-Server IP pair.
But having "only" tens of thousands of connections per client is rarely a problem in practice, apart from some load testing scenarios (such as the experiment here, where they opened a number of ports so they could test a large number of connections with a single client machine).