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What is the meaning of port 0 in netstat -an output in Windows? I'm asking about socket in Foreign address column:

0.0.0.0:0

I know that 0.0.0.0 address means, that host is listening on all interfaces, but what is the function of port zero?

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If you see this as the "Foreign" address, the meaning is slightly different – it's not about the interface; that would be the "Local" address.

All zeros in the "Foreign" column just mean that there is no remote address yet. The line represents a listening socket, which will accept connections from any address, any port. Those connections would be displayed as separate lines.

(From what I remember, in some early systems it used to be possible to create TCP sockets that would only accept connections from a specific host, a specific port, or both.)

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  • So to sum up: port 0 means ANY port?
    – Rafał Ryszkowski
    Commented Feb 6, 2012 at 8:03
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    Yes. In the Foreign address, 0:0 means any remote host, any remote port. (Just to remind: this column is not about local interfaces.)
    – grawity_u1686
    Commented Feb 6, 2012 at 8:34
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    @RZR A TCP listening socket or any UDP socket will basically always be listening for connections or datagrams from any port.
    – David Schwartz
    Commented Feb 6, 2012 at 9:56
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    … although a connect() on a UDP/IP socket will change this behaviour.
    – JdeBP
    Commented Feb 6, 2012 at 13:10

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