Automatically assign DNS How do I ensure that the DNS is being set by the network?

On Windows I would check here and make sure that Obtain DNS server address automatically was selected:
Open System Preferences and click the Network icon.



Select your network in the list on the left, and make sure that it says "Using DHCP" in the drop-down list after "Configure IPv4:"



If you've made any changes, click the "Apply" button and then the "Show All" button to make sure everything is saved.