I could be vary wrong but i think it is by default. I have this simple NTP config
ntp clock-period 17180322
ntp update-calendar
ntp peer 192.5.41.40
ntp peer 199.165.76.11 source FastEthernet0/1/0 prefer
and we point all our SUN and WinBlows boxes at it and they poll from it.
Just try pointing to the IP of your ethernet interface and see what happens.