The FBI is pressing Internet service providers to record which Web sites customers visit and retain those logs for two years, a requirement that law enforcement believes could help it in investigations of child pornography and other serious crimes.

If logs of Web sites visited began to be kept, they would be available only to local, state, and federal police with legal authorization such as a subpoena or search warrant.

What remains unclear are the details of what the FBI is proposing. The possibilities include requiring an Internet provider to log the Internet protocol (IP) address of a Web site visited, or the domain name such as cnet.com, a host name such as news.cnet.com, or the actual URL such as http://reviews.cnet.com/Music/2001-6450_7-0.html.

While the first three categories could be logged without doing deep packet inspection, the fourth category would require it. That could run up against opposition in Congress, which lambasted the concept in a series of hearings in 2008, causing the demise of a company, NebuAd, which pioneered it inside the United States.

The technical challenges also may be formidable. John Seiver, an attorney at Davis Wright Tremaine who represents cable providers, said one of his clients had experience with a law enforcement request that required the logging of outbound URLs.