Rogue IPV6 DNS entry in IPV4 config

I'm a new comcast customer and deployed a fairly simple netgear NAT behind the CG3000DCG gateway and have been in DNS hell for a few days now.  In addition to the official 75.75.75.75 DNS entries I supplied to the gateway and the NAT behind it,  I get a bizarre 2001:4888:61:ff00:604:d::  dns entry handed out by my DCHP sever on the NAT.   It appears to come from the gateway and it's screwing up all DNS lookups because that IPV6 address isn't a dns server.  I have no idea where the entry is coming from but I'm on day 3 of troubleshooting and getting nowhere.   I have temporarily set the DNS entry on the DHCP clients to a static 75.75.75.75 so work can proceed but have no idea where that address is coming from.     Nothing on the gateway mentions it and nothing on the NAT or any other client on the system.  It appears to be in a verizon block somewhere on the east coast.... any ideas?

Darren