I too use Airport Extremes. My recommendation, if there is no compelling need for IPv6 on your network (and there rarely is), disable IPv6 by selecting Link-Local. This will greatly simplify your network management and avoid a number of potential IPv6 issues that may puzzle you greatly.
I run my network IPv4 only (have forever) and have never had any problems due to lack of IPv6.
My domain name defaults to blank - but I don't use Comcast. I'm with Cox. The difference may also be related to IPv6 on in your case and off in mine.
