Port 0

Port 0 is officially a reserved port in TCP/IP networking, meaning that it should not be used for any TCP or UDP network communications.

However, port 0 sometimes takes on a special meaning in network programming, particularly Unix socket programming. In this environment, port 0 is a programming technique for specifying system-allocated (dynamic) ports.

Instead of "hard-coding" a particular port number, or writing code that searches for an open port, Unix programmers simply specify port 0 as a connection parameter. That triggers the operating system to automatically search for and return the next available port in the dynamic port number range.

This programming technique does not work the same way in Microsoft Windows as it does in Unix.