It appears that my AT&T U-verse 2Wire Router was getting bored because I haven’t paid it any attention in the last 90 days, so it decided to screw with me over the last couple of days.
I’m guessing that 2Wire sent another firmware update without notification and it messed-up the DHCP settings.
It appeared that everything was good on the 2Wire side, because I could plug my laptop in directly and it worked fine. That inferred that the problem was with a downstream router (I utilize a “router-behind-a-router” network to reduce domain collisions and increase security), so I performed a power cycle on the unit, and when that failed to correct the issue, did a Factory Reset. It still didn’t work, so I plugged in a spare router, still nada. This was good evidence that the upstream 2Wire really was the cause of the problem.
So I went back to the 2Wire router and looked around. Then I saw it: the DHCP Mask, which is usually 255.255.255.0. But on the 2Wire, it was 255.255.252.0. Doh!
AT&T seems to be mucking around with my customized settings….