Minimal .ssh/config for passwordless login

The following configuration specifies an SSH key and username to login to a specific machine.

Host 192.168.1.215
  User uli
  IdentityFile /home/uli/.ssh/id_bup
  IdentitiesOnly yes

After you have copied it to ~/.ssh/config, you can just

ssh 192.168.1.215

to the machine, without specifying key or user name