Comcast customers can already connect to free WiFi on their laptops and phones in hundreds of thousands of locations across the country. Now Comcast is expanding its free WiFi network by turning home routers into public hotspots.

It’s a rare case of a large company doing something good for its customers that it’s not required to do – no immediately obvious evil motive, nothing that justifies cynicism. It’s a little-known benefit of being a Comcast customer and the company deserves full credit for adding value and providing an incentive to sign up with it for home or business Internet service.

A year ago, Comcast embarked on an ambitious project to turn on free wireless access for Comcast customers across the country. The Bay Area is blanketed with Comcast wireless from access points near business Internet subscribers. Now Comcast has begun broadcasting a public signal from residential routers – starting on the East coast, most recently in Houston and Chicago, with plans to expand rapidly across the country.

If you see a hotspot named Xfinity WiFi on your laptop or phone, you are within range of a Comcast wireless access point. The hotspots are only for Comcast customers; the first time you connect, you will be taken to a web page for your Comcast account credentials.

Behind the scenes, Comcast records a bit of information about your device – probably the MAC address of the network adapter, which is unique for every single network adapter in every single device. Comcast remembers that device and will never again prompt for credentials to join an Xfinity WiFi network from that device. If you open your laptop in a different location and an Xfinity WiFi hotspot is nearby, you’ll be connected. Technically it’s an impressive and elegant way to make this work. It also means that Comcast can track the movements of that device – worth knowing if that sort of thing disturbs you.

The Comcast project began with wireless access points on outdoor public poles or on separate equipment installed on the premises of business customers. As a result, Xfinity WiFi hotspots are available all over Sonoma County and the Bay Area, as well as in cities across the U.S. Click here for a map of Xfinity WiFi hotspots. There are more details in this article about the status of the project last year.

The next phase involves residential service. Comcast has been placing routers in homes that can broadcast two completely different wireless networks – different SSIDs, different IP addresses, isolated from each other.

The customer’s wireless network is controlled by the customer, just as you’d expect. You can name the network and set the security key on it.

Separately, though, Comcast can activate a second network named Xfinity Wifi from the same router. It does not interfere with your Comcast service in any way; it does not affect your bandwidth and you’re not responsible for anything done on that network. Comcast claims it has enough extra capacity that the guest wireless traffic through the same router won’t affect your connection speed at all.

Your signal won’t blanket the neighborhood. This is primarily for people in your house. Comcast has created a guest network – a connection for guests to use so you don’t have to connect them to your network and give up your security key. You also won’t run the risk that your guests or neighbors will hack into your computers. You’d like to think your guests wouldn’t do that, but better not to have to worry about it.

After test rollouts, Comcast turned on hundreds of thousands of residential hotspots in Chicago in March, and is adding 150,000 routers in Houston this month. The plan is to activate 8 million hotspots by the end of 2014. I haven’t found any information about whether this has begun in California but keep in mind that business locations in many California cities are already well-covered from the earlier phase of this project.

There is some resistance to this. Some people find it creepy that Comcast would turn on a network in their houses without asking permission. We are forced to rely on Comcast’s promises about the network architecture to believe the guest network is secure and that our bandwidth will not be compromised. There will be people who buy their own routers to replace Comcast equipment so the free WiFi can be thwarted.

Personally, I’m still tickled when I open my laptop and discover I’m online without having to search out a wireless network or turn on Internet Sharing on my phone. It seems like a good thing to me. If you’re a Comcast customer, figure out your Comcast login and password in case you need them!