Brute forcing hashes on ssh known_hosts

While developing on Fujitsu K5, i realised that i was filling my known_hosts with IPs.
In my ssh_config now, for the address spaces i dont need:

  UserKnownHostsFile=/dev/null
  StrictHostKeyChecking=no

Then, it says i adds to known_hosts, but it doesn't

$ ssh -F k5_ssh_config 10.77.1.3
Warning: Permanently added '62.60.42.151' (ECDSA) to the list of known hosts.
Warning: Permanently added '10.77.1.3' (ECDSA) to the list of known hosts.
Welcome to Ubuntu 14.04.5 LTS (GNU/Linux 3.13.0-61-generic x86_64)

  Here is my gist: