Inconsistent octal madness

$ ping 010
PING 010 (0.0.0.8) 56(84) bytes of data.
$ ping 010.1.1.1
PING 010.1.1.1 (8.1.1.1) 56(84) bytes of data.
$ ping 018
ping: 018: Name or service not known
$ ping 18
PING 18 (0.0.0.18) 56(84) bytes of data.
$ ping 018.1.1.1
PING 018.1.1.1 (18.1.1.1) 56(84) bytes of data.
$ ping 18.1.1.1
PING 18.1.1.1 (18.1.1.1) 56(84) bytes of data.