By default the Hostname on a Mac will be changed by DHCP. Most of the time, I want my hostname to stay the same. So this can be overridden by setting it in:

/etc/hostconfig
change:

HOSTNAME=-AUTOMATIC-
to read
HOSTNAME=mymac

ref: http://hintsforums.macworld.com/showthread.php?t=29712

Set it in the Terminal with
sudo scutil --set HostName <putinyourhostname_or_fqdn_here>
like in: sudo scutil --set HostName server1.mynetwork.com

OR

   1. System Preferences.
   2. Network
   3. Select your network adapter on the left.
   4. Select "Advanced" button at the bottom.
   5. Set the "DHCP client ID" to your hostname.

ref: http://superuser.com/questions/49891/how-can-i-stop-mac-os-x-overriding-my-hostname-when-i-receive-a-dhcp-request-on-s