It was designed for testing, debugging, measurement, and host-management purposes.
For this reason, UDP is usually used, but TCP allows the services to be accessible on session-oriented connections (for example via HTTP proxies or some virtual private network (VPN)).
On most Unix-like operating systems a discard server is built into the inetd (or xinetd) daemon.
It may be enabled by adding the following lines to the file /etc/inetd.conf and reloading the configuration: The Discard Protocol is the TCP/UDP equivalent of the Unix file-system node /dev/null.
On various routers, this TCP or UDP port 9 for the Discard Protocol (or port 7 for the Echo Protocol relaying ICMP datagrams) is also used by default as a proxy to relay Wake-on-LAN (WOL) magic packets from the Internet to hosts on the local network in order to wake them up remotely (these hosts must also have their network adapter configured to accept WOL datagrams and the router must have this proxy setting enabled, and possibly also a configuration of forwarding rules in its embedded firewall to open these ports on the Internet side).