Comcast's rental cable modems are a rip-off. It's much cheaper to buy your own.

One of Comcast's most ludicrously profitable businesses is renting out cable modems. These devices, which allow users to access the internet over cable networks, can cost as little as $30 if you buy them used. And until recently, Comcast was renting them out to people for around $8 per month. In other words, as little as six months of modem rental fees cost more than buying a device outright.

Now Ars Technica reports that Comcast is raising its cable modem rental fee in many parts of the country to $10 per month. It might be time for you to buy your own cable modem and stop paying Comcast's inflated rental fees.

I did this back in May. It was easy, and I've already saved more than the cost of the modem. You can do it too.

My cable modem cost just $37.94

The scene when I visited my local Comcast office in Washington, DC, to return my modem on September 13, 2014. (Timothy B. Lee/Vox)

Getting my own cable modem was surprisingly easy. Here's how I did it. I looked at the cable modem Comcast had provided me and found the model number. A quick web search revealed that the exact same model was available on www.buyyourownmodem.com — refurbished and with a six-month warranty — for $30.95 plus free shipping and handling.

Once I got the modem, setup wasn't too difficult. I called Comcast and told them I bought a new modem. They walked me through the setup process and then made arrangements to return the old device.

Returning the old modem was the most frustrating step. The Comcast representative promised to send me a shipping box, but the box never arrived. So after waiting for two months I decided to go to the local Comcast office. The place was mobbed, and I wound up waiting in line for more than 30 minutes to return my device.

Still, the whole process — ordering the modem, installing it, and returning the old one — only took a couple of hours. I've already saved more in modem rental fees than I paid for the modem, and I may be able to use this same modem for years to come.

You can avoid modem rental fees too

I got a good deal. The modem I purchased retails for $69.95 on www.buyyourownmodem.com. The refurbished price has risen to $55 from the $30.95 I paid last year. And you might need to pay more if you need a modem that supports features such as voice calling.

GETTING MY OWN CABLE MODEM WAS SURPRISINGLY EASY

Still, most of the devices on Comcast's list of approved modems are available for less than $60 if you buy them used or refurbished. Some Comcast-approved modems are available used for less than $20.

A foolproof way to make sure you get the right modem is to search www.buyyourownmodem.com for the exact same model of modem you currently have. Mine, for example, was an Arris CM820. That minimizes the risk of getting one that's not compatible with your current service.

Getting your own modem does cost money up front, and it's a bit of a hassle. But with Comcast's modem fee rising to $10 per month in many parts of the country, you'll make that cash back in a few months.

And the same advice likely applies for other cable companies. Vox's Todd VanDerWerff, for example, is a Charter customer, but he also saved money by buying his own modem.

Potential pitfalls to buying your own modem

After I published this story, a former Comcast tech wrote in to note that buying a used cable modem can be a hassle. That's because if a modem was previously registered to another Comcast customer, then Comcast will have to contact the previous customer to get the modem de-registered before registering the modem with a new customer.

That's not the end of the world, and it might not be a problem at all (I bought a refurbished modem with no problems). But you could save yourself some hassle if you buy a new modem — and this is still likely to be a cheaper option than a year's worth of modem rental fees.

The other potential pitfall is that tech support issues might be harder to deal with if you own your own modem. With a rented modem, your cable company will replace a defective modem free of charge. In contrast, if you own your own modem, the cable company might blame your modem for the problem and ask you to buy a new modem yourself. On the other hand, I've talked to a number of people who have purchased their own modems and haven't encountered this problem.