Beginning Oct. 1, Comcast will limit use of Internet bandwidth - a measure of all a person's monthly downloads and uploads of text, graphics, music, movies, photos and other information. The company said it reserves the right to terminate any residential customer who disregards company warnings and twice violates a limit of 250 gigabytes per month. Previously, the company didn't have a specific limit.

The idea is to keep bandwidth hogs from ruining the Internet experience for other residential customers, because the cable network is a shared medium of limited bandwidth capacity, Comcast said.

But that has some Comcast customers worried. Comcast provides about 100,000 Anne Arundel County customers with telecommunications services.

"Your typical Comcast Internet user will not be affected, but the power users - people that watch movies or TV online, or that download a lot of video or software - will be hurt," said Ryan Coleman, a Comcast Internet customer in Minneapolis who edits photos for a college magazine and downloads 40 to 80 gigabytes of data every week. "I have a feeling that I might be dead in the water in October."

Others are concerned about what this means for the Internet's future.

"It's absolutely critical that the Internet remain a level playing field, and that no one have control over what runs over it," said Steve Borsch of Eden Prairie, Minn.

Mr. Borsch runs a business and blogs about technology using Comcast's Internet service. He said the bandwidth limit is a bad move because it hurts Internet video services while helping Comcast's cable TV service.

Some analysts agree.

"A bandwidth limit discourages consumers from downloading or streaming Internet video, particularly of high-definition video," said S. Derek Turner, research director for Free Press, a Massachusetts-based nonprofit public policy group dealing with Internet and media issues. "That helps preserve Comcast's traditional cable TV service."

Comcast said such concerns are overblown. The company's monthly bandwidth limit is so high - the equivalent of downloading about 62,500 songs - that fewer than 1 percent of its customers are likely to be affected, said spokesman Charlie Douglas at Comcast's Philadelphia headquarters. Today, the average Comcast customer uses only a tiny fraction of the limit, he said, or about 2 to 3 gigabytes a month.

But Mr. Turner said the bandwidth limit is likely to affect more Comcast Internet users in the near future because of products such as Apple TV that can transfer Internet video to television sets.

"Comcast's 250-gigabyte bandwidth cap, while very high now, won't be high in the future," Mr. Turner said. "The way the Olympics were viewed on the Internet signals that consumers are ready to embrace online content."

Even today, someone watching eight hours a day of a standard-definition video or four hours a day of high-definition video could run afoul of Comcast's bandwidth cap, Mr. Turner said.

Mr. Douglas said Comcast will periodically review the bandwidth limit as online video viewing increases, but he declined to say whether the company would increase it.

Comcast has said that the bandwidth cap is unrelated to a recent Federal Communications Commission finding that Comcast was improperly limiting the ability of its customers to use BitTorrent, a popular file-sharing program. On Thursday, Comcast said it was appealing the FCC's ruling.