Disable IPv6!!!

IPv6 breaks things! We don't need it and don't want it, but Comcast denies customers the ability to disable it. NO HOME USERS NEED IPv6, and few if any business customers actually need it. Certainly not on the LAN interface. 

Comcast gateway/modem/routers provide NAT between IP versions 4-4, 6-6, and 4-6, so IPv6 on the LAN is absolutely unnecessary. It just creates problems. If you don't already know this  you're not paying attention.

Your arrogance costs your customers innumerable hours of frustration and downtime. STOP IT! 

FYI 

I am an IT network consultant of over 40 years, I've published books on networking and internetworking and many articles in many publications over decades, as well as conduct thousands of networking seminars internationally.