I know, I know, it sounds like a ThioJoe video. I assure you this is not.

Recently, I switched cable modems with my ISP. Let’s say this ISP was AS12083. In fact, I switched my cable modem over today! When I called customer service to transfer over my modem, it went over without a hitch, as the new config was transferred over TFTP to the new modem and rebooted. I unplugged the old modem and switched over to the new one. The first thing I noticed was that I was not receiving two public IP addresses as I did before, but that was not an issue.

During the night, I wondered what would happen if I connected my old modem back up to the coax cable and powered it on. Would it work? Or would nothing happen? I had to find out. I plugged the old modem back up, with the new one still connected, and connected my laptop to the old modem. After a few seconds, all the lights on the old modem were lit up and solid. For reference, the old modem was a SB6121 and the new one is an Arris TM822G. I connected my laptop to the old modem, and I received a DHCP address in the 10.0.0.0/8 range. Whenever I navigated to a page, it would not load, and I could not ping anything outside of my laptop, not even the gateway it gave me.

After I established that I had no internet, I tried connecting to one of my ISP’s servers, which would display a message saying that the cable modem was not provisioned. This IP was http://75.76.84.104/, and sure enough, it loaded just fine! This got me thinking to maybe there was another way to connect to the internet. I tried pinging one of my servers within the same ISP. When I tell you my heart dropped when I saw the ping response come back as positive, it dropped.

When I found out that I could ping my server, which was on a public IP address on the same ISP, I got to thinking. What would happen if I tried to establish a VPN connection to this server? I pressed connect, and macOS said a connection was established. I tried going to Google, and it worked. I found a way to connect to the open internet without paying for service. I checked my server, and oddly enough it showed my laptop as a client with a 10. address!

However, there are caveats to this that I learned.

  • The only way to interface with the outside world was this VPN
  • The config on the modem limited my computer to 1 Mbps down, and 1Mbps up
  • There must be a VPN server with a public IP on the same ISP’s network.

TLDR

  • Used an unprovisioned modem to technically obtain free internet access, although with some caveats.