Bandwidth hogs may need to start pulling back on their freeloading ways. Internet service providers are cracking down on their most active subscribers.

Following the lead of Cox Communications, which has been limiting the amount subscribers can download and upload since 2005, Comcast ()CMCSA recently announced it would impose an official limit of 250 gigabytes per month on its customers. (See our table for individual companies' usage caps.)

The ultimate goal, it appears, is to turn the traditional all-you-can-download system into an a la carte one, where the heaviest users end up paying more for their big bandwidth appetites. That poses a potential problem for subscribers who are using a rapidly increasing amount of bandwidth to do such things as download movies and play multiplayer videogames, says Phillip Dampier, editor of StoptheCap.com, a consumer site that opposes unfair broadband usage caps. "Right now 250 GB sounds really generous, but in two years from now, when everyone will be watching [high-definition] TV over the web, it doesn't sound so good," he says.

The moves beg the question: Will your connection get cut in the middle of downloading an episode of "Ugly Betty" because you exceeded your monthly limit? Here's what consumers need to know.

Who the caps affect

Just how large is 250 GB anyway? Comcast tells subscribers that they'll hit that limit if, for instance, they download 125 standard-definition movies (at 2 GB/movie) or download 62,500 songs (at 4 MB/song).

"There aren't many people getting anywhere near this cap," says Dave Methvin, chief technology officer of PCPitstop.com, a site that conducts diagnostic tests on personal computers. "Most likely the people to be caught by [Comcast's] cap are those doing a lot of file trading or downloading large amounts of data."

A one-computer household likely won't hit that ceiling. But if everyone in the family has a laptop chances become much higher. If one kid plays videogames, while Mom watches television online and Dad does regular backups of his PC files, total usage can get close to that 250 GB limit, says Tom Merritt, executive editor at CNET.com.

Those surpassing Comcast's cap won't get hit with overage fees, but they do risk getting cut off. "It's sort of a proxy for saying 'if you're downloading 250 GB, you're not the kind of user we want on our network anyway,'" says Drew Clark, executive director of BroadbandCensus.com, a broadband information web site.

Comcast's limit is generous compared to Cox's, which requires subscribers to sign up for high-speed packages with varying caps. The cheapest, at about $30 a month, buys a maximum of 4 GB -- downloading a few high-definition movies would get a subscriber into trouble with this plan. The highest amount Cox offers is 75 GB and runs about $55 a month. Cox's policy says it can terminate or suspend a customer's account if they exceed their limit. However, a company spokesman says "those are a last resort and only a tiny percentage of customers have ever reached that level of excessive use."

How to gauge usage

To figure out how close you are to reaching your cap, download software that tracks usage. Technology news and review site CNET recommends BitMeter, which allows users to monitor their Internet connection and see usage over the last few hours, days or weeks. It also alerts users when they're coming dangerously close to their bandwidth limits. Another application CNET likes is SurplusMeter, which provides statistics on such things as the number of megabytes used. Alan Stafford, editor of PC World magazine, recommends that consumers with bandwidth constraints use ShaPlusTo read CNET's reviews, click here

Cap-free alternatives

Not every ISP is enforcing bandwidth caps. Some providers, like Cablevision, Verizon ()VZ FiOS and Windstream DSL don't carry limits -- at least not yet, says Dampier.

FiOS, for example, is an excellent high-speed alternative, says Merritt. It's able to offer faster Internet speeds than DSL or cable modems because the fiber goes directly to the home rather than being converted to coaxial cable that's shared among neighbors. Available in 16 states, Verizon offers bundled FiOS services that include Internet, TV and phone that are competitively priced (plans start at $43 a month.) One caveat: While installation for one computer is free, Verizon charges a pricey $89.99 to set up each additional computer.

To do your own comparison shopping, check out Broadbandreports.com, which lets consumers search for providers in their area and compare offerings.

* Source: Companies' spokespeople and web sites of Cox Communications, Comcast and Time Warner Cable.

Corrected Sept. 5, 2008:
In the original story we stated that AT&T is planning to put caps on the amount of bandwidth its high-speed Internet subscribers can use each month. AT&T is not implementing such a cap.