Comcast's decision to put a 250-gigabyte cap on monthly bandwidth use for its residential customers may look like a generous number, but some critics suggested the cap may cause problems for users in the future.

Comcast announced late Thursday that starting Oct. 1 it will have a 250G-byte cap on monthly residential bandwidth, with the broadband provider saying it may send warnings to subscribers who go over the limit. If a subscriber goes over the limit a second time within a six-month period after getting a warning, Comcast will suspend the customer's account for a year.

Less than 1 percent of Comcast users will be affected by the limit, and customers who go over the 250G-byte cap will not automatically be warned, depending on whether they are in that top 1 percent, said Charlie Douglas, Comcast's director of communications. In the past, Comcast did warn high-bandwidth users and cut them off after a second warning, but the company did not have a firm bandwidth cap.

The announcement leaves many questions unanswered, said Art Brodsky, communications director for Public Knowledge, a digital rights group that's been critical of other Comcast network management efforts.

"At the moment, it seems relatively benign, but there are dangers lurking," Brodsky said. "Will Comcast count some traffic against the cap, but not other traffic -- as in Comcast-generated video?"

It's unclear whether Comcast traffic, including its VoIP (voice over Internet protocol) service, would be counted in the cap. Douglas didn't immediately return a phone call seeking a clarification on that issue.

But in an earlier interview, Douglas defended the cap, saying most Comcast customers use less than 3G bytes of bandwidth a month. "We think 250 [GB] is an extremely large amount of bandwidth," he said.

Other broadband providers have looked at bandwidth caps as well, with some considering caps as low as 5G bytes per month. At least two other providers in the U.S. and Canada have instituted caps, Douglas said.

The bandwidth cap doesn't appear to help with network congestion, Brodsky added. "A low-bandwidth user downloading an HD movie at 8 p.m. in peak time puts more stress on the network than someone who downloads lots more in the middle of the night," he said. "If fewer than 1 percent would be affected by this, what's the point?"

Brodsky and Om Malik, founder and senior writer at the GigaOm technology blog, also questioned how Comcast customers will know if they're getting close to the limit. Comcast has advised customers to search online for bandwidth monitoring tools if they want to know how close they are to the limit.