Comcast installed a SMC 8014 modem (link to the manual) for my business internet. My default admin login is http://10.1.10.1. The default username/password is:
cusadmin/highspeed.
I also have a Motorola SBG900 modem (link to the manual). The default username/password is:
admin/motorola
PS: It is simply terrible that this info is so difficult to find.