NEW YORK — Most home Internet service providers offer unlimited data, but cable giant Comcast is moving in the opposite direction.

It’s started charging heavy Internet users extra in more parts of the country.

The reason? A small but growing number of consumers are skipping cable subscriptions and doing their TV-watching over the Internet instead. So finding a way to charge for heavier Internet use could bolster Comcast’s revenue as the ranks of its cable customers shrink.

Comcast actually used to impose a monthly 250 gigabyte data cap on its customers, but ended it in May 2012 to experiment with alternative ways of managing bandwidth. That August, it capped monthly data use for Nashville, Tenn. customers at 300 GB; going over the limit cost $10 for every 50 GB. The company launched a similar plan in Tucson, Ariz., that October — you got 300 GB for a base plan, 600 GB if you signed up for a faster and more expensive connection.

By December 2013, Comcast had rolled out the Nashville system to Atlanta and a handful of smaller markets, many in the South. It also offered a slow Internet plan of 3 megabits per second that gave you a $5 credit if you used 5GB or less each month, and charged you $1 for each gigabyte of data over 5 GB.