Just to be sure, with a routed network being aimed at your house, you'll actually receive two subnets (well, a single address in one subnet, and a whole second subnet). You need to use DHCP-PD or similar to make use of it.

The /128 sounds perfectly normal, and is used as a "point to point" link between the ISP and your modem. Over this, DHCP prefix delegation typically gives you a whole second subnet of /60 or so. If your modem isn't handling DHCP-PD, then all your see is 1 measly address even though they've likely allocated and pushed 295,147,905,179,352,825,856 addresses to you.

Hmmm, maybe I should revisit this. I first tried it a year or so ago with pfSense on a virtual machine. Between the IPv6 issues and generally poor performance (long story), I went back to my IPv4-only configuration with DD-WRT on a D-Link DIR-825.

I'm now using a Ubiquiti EdgeRouter POE, and maybe I should give it another shot. I'm not convinced I'll actually get a prefix from Comcast, though.

It may differ by region, but in the SF Bay Area I've been getting a /60 from Comcast residential for a few years now using a MikroTik router. See here: http://i.imgur.com/2mrRk01.png

The edge router can do DHCP-PD, but it needs some configuration for it to do it, there's lots of info on the Ubuiquiti forums about it. Good luck!

Comcast has a decent ipv6 technology group (by necessity - they were an early adopter/innovator of carrier-grade v6 as they have more CPE to address than can be fit into 10/8). If you ever become interested in resolving this you may want to dig through http://www.comcast6.net and see if you can find a direct contact to work with.

I don't have any contacts myself, I'm just a customer and interested observer.