I recently solved a problem that’s been bugging with me on Comcast HSI (High Speed Internet), and just in case this helps anyone else, I wanted to document it.
In my case, I’d first plugged in the cable modem, then my Macbook, and gotten zero. The Mac had been issued a DHCP address that looked OK, but no internet access. It got as far as “looking up XXX”. This indicated that DHCP had told the computer where the DNS server was, but my laptop couldn’t actually talk to it.
I gave up after a while, and turned off the modem. I tried again the next day, and it worked.
Later, I wanted to get my Airport Express to work. But after plugging the Airport Express into the network, it couldn’t get a DHCP address.
After poking around online, I discovered that it was possible to log onto the web interface of my cable modem (a Thompson TCM 420). (You can do this by getting onto the same network, setting a static address of 192.168.100.2, and browsing to 192.168.100.1.) Strangely, it reported that only one network device was supported. So, I unplugged my Mac, just plugged in the Airport, and reset everything. Still no dice.
I gave up after a while. But this time, I had a suspicion. I turned off the modem for about 6 hours, then turned it back on with only the Airport Express plugged into the wired network. Bingo!
Here’s my stab at what’s happening. I believe that Comcast really is allowing only one DHCP address to be issued to devices on my network, and that it remembers the last device that was issued (like a DHCP reservation). If a new device appears on the network, it only allocates an address to the new device if the old device’s DHCP reservation has expired, which appears to happen after a few hours.
In the first case of the non-working DHCP address, that was bad luck. When the computer’s reservation expired and it was granted a new IP, the new one worked.
… appears to be that if you’re messing around with your network, you need to turn off your modem for a fair period of time to allow old leases to expire and new devices to get new IPs.