Port 0 (Reserved)

TCP Reserved

UDP Reserved

SCTP —

DCCP —

What it is

In programming APIs, requests a system-allocated (dynamic) port; never used on the wire.

Bind to port 0 in code to let the OS pick a free ephemeral port.

Transport	Status	Meaning
TCP	Reserved	Reserved by IANA, generally to prevent collisions after a withdrawn assignment.
UDP	Reserved	Reserved by IANA, generally to prevent collisions after a withdrawn assignment.
SCTP	—	Not applicable for this transport.
DCCP	—	Not applicable for this transport.

1 TCPMUX »

Well-known (0–1023). System port for a core service. On Unix-like systems, binding it requires superuser privileges.

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