What identifies a TCP connection, and how can a server accept 10,000 connections on port 443 when port numbers are only 16 bits?

A TCP connection is identified by its four-tuple. These are four values:

• source IP
• source port
• destination IP
• destination port

If you add the protocol, it becomes a five-tuple. A port alone doesn't identify a connection.

The server has one listening socket. A socket is the OS object a program uses to send and receive network data. The listening socket is bound to an address and port, for example 0.0.0.0:443. Each call to accept() returns a new connected socket for one client. All these sockets share local port 443. But each one has a different remote IP and port, so every four-tuple is unique. The real limits are memory and file descriptors, not ports.

The bind address matters:

• 127.0.0.1 accepts connections only from the same machine.
• 0.0.0.0 (or :: for IPv6) accepts connections on every network interface.

Clients use a temporary ephemeral port for their end of the connection. This is where port limits really cause problems.