The Discard protocol (Discard) is a "testing" protocol, used to verify proper transmission to a server. If contact is established, the server internally notes reception, then discards the packet. The protocol can be used to verify 'send' capability, with the server acting much like a 'NULL port'. Interestingly, most versions of Microsoft Windows contain a built-in Discard server that can be easily activated. Discard servers can be accessed using a TelNet client. Below are addresses to various free public access Discard servers.
[IANA Port assignment = 9]