i cannot understand sshd behavior

["Gustavo Rios" <rios.gustavo@gmail.com>]

I have the following sshd_config (relevant part only):

GSSAPIAuthentication yes
#GSSAPICleanupCredentials yes
KerberosAuthentication yes
KerberosGetAFSToken yes
KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes


When i try to connect to a server by its fqdn no password is request, like in:

$ ssh -l sioux foo.my.domain
Last login: Mon Mar 12 13:18:22 2007 from 10.0.0.250
Naeser's Law:
        You can make it foolproof, but you can't make it
damnfoolproof.
$


But, when i try by IP, i get this:

$ ssh -l sioux 10.0.0.1
sioux@10.0.0.1's password:
Last login: Mon Mar 12 13:21:17 2007 from 10.0.0.250
"The subspace W inherits the other 8 properties of V. And there aren't
even any property taxes."
                -- J. MacKay, Mathematics 134b
$


Why does it happens? I believe i told sshd explicitly not to request
password, didn't i?

Thanks in advance.