Finally, it is even possible to omit the host name part of a
tramp file name. This case, the value of the variable
tramp-default-host is used. Per default, it is initialized
with the host name your local Emacs is running.
If you, for example, use tramp mainly to contact the host ‘target’ as user ‘john’, you can specify:
(setq tramp-default-user "john" tramp-default-host "target")
Then the simple file name ‘/ssh::’ will connect you to John's home directory on target. Note, however, that the most simplification ‘/::’ won't work, because ‘/:’ is the prefix for quoted file names.