How to Buy a Cable Modem for Broadband Internet

Save money by buying rather than renting

A cable modem being set up.


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Cable modems connect a home network to the residential cable line of an internet service provider. These modems plug into a broadband router on one end, typically via either a USB cable or an Ethernet cable, and a wall outlet (leading to the residence's cable feed) on the other end. Cable internet service providers rent these to their subscribers, but you can buy your own cable modem directly. Here's how to find one that's adequate for your needs.

DOCSIS and Cable Modems

The Data Over Cable Service Interface Specification (DOCSIS) standard supports cable modem networks. All cable broadband internet connections require the use of a DOCSIS-compatible modem.

Three major versions of DOCSIS modems exist:

  • DOCSIS versions 1.0 and 1.1 were made available in the late 1990s. They are obsolete now in most parts of the world. These modems support up to 38 Mbps downloads and 9 Mbps uploads.
  • DOCSIS 2.0 supports the same 38 Mbps download speeds as 1.x but increases the maximum upload bandwidth to 27 Mbps. Newer D2.x modems also can support IPv6. (Check the product documentation to confirm).
  • DOCSIS 3.0 and 3.1 support IPv6 and much higher bandwidth connections (over 100 Mbps) than D1.x/D2.x. They are also backward compatible with older DOCSIS versions and networks.

A D3 modem is appropriate for modern cable internet. Although prices for new D3 modems can be higher than for older versions, the price difference has lessened substantially in the past few years. D3 products will provide a much longer useful lifetime than older versions, and they might enable higher-speed connections than older modems would, too.

Your upload/download speeds ultimately depend on the limits set by your cable internet service provider and the service tier you've chosen.

When Not to Buy a Cable Modem

Check your internet service's terms of service to make sure the provider doesn't require you to use only provided equipment. Also, if you're considering a change in internet service providers, you might save money by renting for now.

Renting Cable Modems

Buying a cable modem usually saves money in the long run over renting one. In return for providing a unit they guarantee to be compatible, internet providers typically charge at least $5 per month. The unit might also be used, and if it fails completely (or especially, starts acting flaky), the provider can be slow to replace it.

To ensure you buy a broadband modem that's compatible with your internet provider's network, check with friends or family who use the same provider. Online retail and tech help sites also maintain lists of modems compatible with the major providers.

Buy a cable modem from a source that accepts returns, so that you can try out and exchange it if necessary.

Wireless Gateways for Cable Internet

Most broadband providers offer their customers units that integrate the functions of a wireless router and broadband modem into one device. These wireless gateways have built-in DOCSIS modems. Subscriptions to combined internet, television, and phone services typically require using these devices instead of standalone modems. Like standalone modems, they're available for purchase through the usual outlets. Check your provider's website to ensure compatibility.

Is Your Cable Modem Compatible?

Your most important task if you're considering buying your own modem is to check compatibility with your provider.

xfinity router compatibility

Many cable internet service providers provide lists and tools to check if the modem you're considering is compatible with their services. A few examples: