X
- x-token (MIME encoding type), Multipurpose Internet Mail Extensions
- X.25 networks, packet fragmentation, Fragmenting datagrams
- X.400 protocol, Simple Mail Transfer Protocol
- XDR protocol, Presentation Layer, A Data Communications Model
- xinetd (Extended Internet Daemon), The Extended Internet Daemon, Controlling Access with xinetd–Controlling Access with xinetd
- access control, Controlling Access with xinetd–Controlling Access with xinetd
- xinetd.conf file, The Extended Internet Daemon, Looking for Trouble
- security considerations, Looking for Trouble
- xonxoff option (pppd), The PPP Daemon