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A server can use the wildcard symbol ``*'' to underspecify its location in order to service incoming requests from multiple network interfaces. A service such as ftp can be installed only once on a host which is connected to multiple network interfaces. The ftp program can listen on all the network interfaces:
<<*.21> <*.*>>This tuple signifies that the local ftp on port 21 is listening on multiple interface addresses for whatever client processes that wish to connect.
To name a socket that listens on all network interfaces, the Internet address INADDR_ANY must be bound. If a listening port is not specified, the system assigns one. For more information on wildcarding, see ``Internet address binding''.