In the olden days, “555” alerted any viewer to a fake phone number in a TV show or movie. Thankfully, today, there’s no equivalent for fake Web sites.

For a show like CBS’ “How I Met Your Mother,” this has been boon. If a character on the sitcom mentions a fictional Web site or blog, there’s no need for the Web address to remain fictional.

In quick time, the domain name can be cheaply purchased and registered. Suddenly, a one-liner has new digital life. The joke Web site becomes a real Web site and industrious fans follow the trail.

No show has embraced these possibilities more than “How I Met Your Mother.” Created by Carter Bays and Craig Thomas, the show is based on a group of friends in New York (among them is Barney Stinson, played by Neil Patrick Harris). Unlike most shows, it’s narrated entirely in the past tense and full of comical reminiscences; it’s the William Faulkner of sitcoms.

Like its forays into the past, “How I Met Your Mother” expands its comedy into the Web. In its four seasons, it has created a blog for Stinson and more than six mock Web sites.

“That is always the most annoying thing when you watch a show and you hear that ‘555’ and it takes you out of it,” said Bays in an interview. “The Internet is so user-accessible that whenever you hear “www” on a show, you just know that there’s probably something behind it. There’s an extra layer of depth to this show knowing that you can go to a different medium and continue the story.”

It’s a practice that has become increasingly common. The appeal of mock sites was perhaps first widely noticed in 2006 when Conan O’Brien ad-libbed after a manatee mascot sketch on “Late Night.”

O’Brien joked that the mascot had the Web site “hornymanatee.com.” NBC then purchased the address and “Late Night” quickly developed the site, spawning a hit that attracted millions of visitors.

Bays and Thomas first created a mock Web site when they were writers for CBS’ “The Late Show with David Letterman.” With then head writer Rodney Rothman, they created a fake boy band called Fresh Step (named for the cat litter), had them perform on the show as a musical guest and built a Web site for the band.

On “How I Met Your Mother,” Bays and Thomas have even created a staff position for the creation of mock sites, held by their house Web guru Carl MacLaren.

Together, they’ve made a MySpace fan page (www.myspace.com/robinsparkles) for pop singer Robin Sparkles, an alter ego of the show’s Robin Scherbatsky (Cobie Smulders) who enjoyed brief fame for her ’80s hit “Let’s Go to the Mall.” A 3-minute music video has been viewed by more than 430,000 on YouTube.

There’s a site for Barney’s video resume (www.barneysvideoresume.com) and one for a Web address Robin casually mentions in an episode. Its subject can deciphered from the exceedingly long url: www.guyforceshiswifetodressinagarbagebagforthenextthreeyears.com.

There are others too, but perhaps the best is a site supposedly created by a woman (played by Charlene Amoia) scorned by Barney who thinks his name is Ted Mosby (the name of Josh Radnor’s character). On www.tedmosbyisajerk.com, there’s a psychotic, impassioned 22-minute song by Barney’s crushed admirer.

The song was recorded in Bays’ garage with Amoia singing and ad-libbing. It’s a welcome outlet for Bays and Thomas, despite the added pressure of creating more content.

“The great thing about it is with our job, there’s always 22 minutes of TV time to fill,” says Bays. “With the Internet, it’s nice to write for a medium where there’s very little constraint other than the amount of time you have to work on it and the amount of money you have to spend on it.”

Conceived as low-budget larks, the producers are inevitably caught by surprise at the traffic to these sites. Bays says the numbers are usually in the hundreds of thousands.

“We always underestimate how many people are going to tune it,” he says. “Every time we put a Web site up, routinely, it will crash within the first 15 minutes.”

The show will make its latest extension into cyberspace on its March 23 episode, when fans can look for www.Canadiansexacts.org. Bays expects that site and the others to remain active for years.

“Now that our shows are being rerun in syndication and it’s on DVD, you kind of have to account for that and keep the stuff up,” says Bays. “Who knows how long the Internet will be around for before it gets replaced with something else, but for the time being, as long as you hear “www” in an episode, you’re going to want to go there and check it out.”