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### 16.7 Other Namespaces

Certain other namespaces and associated protocol families are supported but not documented yet because they are not often used. `PF_NS` refers to the Xerox Network Software protocols. `PF_ISO` stands for Open Systems Interconnect. `PF_CCITT` refers to protocols from CCITT. `socket.h` defines these symbols and others naming protocols not actually implemented.

`PF_IMPLINK` is used for communicating between hosts and Internet Message Processors. For information on this and `PF_ROUTE`, an occasionally-used local area routing protocol, see the GNU Hurd Manual (to appear in the future).