Thunderbolt Ethernet has a self-assigned IP address and will not be able to connect to the Internet

Getting this error when I try to connect my Macbook Pro Retina 15-inch (Mid 2015) to an ethernet line:

"Thunderbolt Ethernet has a self-assigned IP address and will not be able to connect to the Internet"

My ethernet cable is connected to a 7-port switch, which connects to a Motorola modem, which connects to Comcast broadband internet... Have tried turning the **** thing on-and-off a bunch of times already (re: No, this is not my first IT question).