Some providers require a hostname in the DHCP settings. It is not believed that Comcast requires this. If you are having a problem, try the following steps in the exact order listed.
Disconnect the Ethernet cable from the SonicWall to the modem.
With the SonicWall disconnected from the modem, set up the X1 (WAN) interface to use NAT with DHCP Client (SonicOS Standard or Firmware 6.x) or DHCP (SonicOS Enhanced). Leave the hostname blank.
Power off the SonicWall.
Unplug the cable modem and disconnect the co-ax cable. If the modem includes battery backup, remove the battery.
Connect the Ethernet cable from the SonicWall (still powered off) to the modem.
Power on the SonicWall.
Reconnect the co-ax cable to the modem.
Plug in the modem power cord. If the modem includes battery backup, insert the battery.
Connect your laptop or PC to the X0 (LAN) interface.
If these troubleshooting steps have been exhausted without success, make sure you are running the latest firmware release and upgrade if you are running an older version. You should now have a successful Internet connection.
NOTE: This procedure is based on user experience and testing, and is provided for guidance purposes only.