The meaning depends on the context of use, but IP address 0.0.0.0 is the
address, and 3965 is the port assignment. If you are looking at the
value of a socket address being passed to bind() before a listen(),
this socket address means attach to the wildcard IP address on local port
The networking stack will bind the IP address to the IP address of
one of the configured active interfaces.
The meaning of the port assignment may be pre-established or
made up by convention between the service and clients.
To see the assigned port numbers for a particular machine to see what
service listens on that port, look at the /etc/services file. To know the
pre-assigned port numbers for a wide variety of standard services and some
ones, check the IANA assigned port number registry: