Follow these instructions to ensure you are using DHCP when connecting to a network on a Mac. Pictures are attached below to assist you.
  • Click on your wireless settings and click on Open Network Preferences...
  • Click on Wi-Fi and then Advanced
  • Click on TCP/IP
  • Verify that it says Configure IPv4: Using DHCP 
  • Double check that it is "Using DHCP" and not "Using DHCP with manual address"