Networks

Edit the interface file, but first make a backup of it

  /etc/network/interfaces. 

Change the eth0 connection to look like this

iface eth0 inet dhcp
auto eth0

Delete the other address/gateway/etc information from that section.

Reload the network settings by doing this:

sudo invoke-rc.d networking restart

That'll grab you an IP address from comcast. It will also put the correct nameservers in resolv.conf for you.