What Is Port 0 Used For?

Port 0 carries special significance in network programming, particularly in the Unix OS when it comes to socket programming where the port is used to request system-allocated, dynamic ports. Port 0 is a wildcard port that tells the system to find a suitable port number.

Unlike most port numbers, port 0 is a reserved port in TCP/IP networking, meaning that it should not be used in TCP or UDP messages. Network ports in TCP and UDP range from number zero up to 65535. Port numbers in the range between zero and 1023 are defined as system ports or well-known ports. The Internet Assigned Numbers Authority (IANA) maintains an official listing of the intended usage of these port numbers on the internet, and system port 0 is not to be used.

Configuring a new network socket connection requires that one port number be allocated on both the source and destination side. TCP or UDP messages sent by the originator (source) contain both port numbers so that the message recipient (destination) can issue response messages to the correct protocol endpoint.

IANA has preallocated designated system ports for basic internet applications like web servers (port 80), but many TCP and UDP network applications do not have their own system port and must obtain one from their device's operating system each time they run.

To allocate its source port number, applications call TCP/IP network functions like bind() to request one. The application can supply a fixed (hard-coded) number to bind() if they prefer to request a specific number, but such a request can fail because another application running on the system may currently be using it.

Alternatively, it can provide port 0 to bind() as its connection parameter. That triggers the operating system to automatically search for and return a suitable available port in the TCP/IP dynamic port number range.

The application isn't granted port 0 but rather some other dynamic port. The advantage of this programming convention is efficiency. Instead of each application implementing and running code to try multiple ports until they obtain a valid one, apps rely on the operating system.

Unix, Windows, and other operating systems vary in the handling of port 0, but the same general convention applies.

Network traffic sent across the internet to hosts listening on port 0 might be generated from network attackers or accidentally by applications programmed incorrectly. The response messages that hosts generate in response to port 0 traffic help attackers learn the behavior and potential network vulnerabilities of those devices.

Many internet service providers (ISPs) block traffic on port 0 — both incoming and outgoing messages — to guard against these exploits.