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Where is the security key on a comcast modem located? The security key on a Comcast Modem is located next to the serial  number of the unit. Sometimes the serial number is the number used  as a security code if it is not printeâ¦d on the device. (MORE) 1 person found this useful.
Look for Default Gateway under the Ethernet adapter listing for the modem's IP address. Type your user ID and password in the login screen. Unless you've changed the login credentials, enter the default values. If you are unable to log in, look up the correct login details in the user manual for your Comcast device.