The Active Network, a network of sites targeting athletic and active-lifestyle users, announced today that it’s buying ReserveAmerica, an online service for booking campground space across the U.S., for an undisclosed amount.

It was formerly a subsidiary of IAC, a conglomerate of interactive web-based businesses (including CollegeHumor, Vimeo and eVite), which agreed to hand over 100 percent of its stock in Reserve America Holdings in exchange for 3.5 million shares of convertible preferred stock in Active. This will give IAC a nine percent stake in the San Diego-based company.

Coordinating 350,000 campsites for more than 17 million customers a year, ReserveAmerica first took its business online in 1997 after 13 years. The acquisition seems to make a lot of sense for Active, which tries to give its members new ways to get involved with sporty activities and events. It will absorb all of ReserveAmerica’s 460 employees, bringing its own total to 250. A spokesperson for the network says the deal, which closes at the end of the month, should strengthen the market position of both parties.

Active last raised funding in August, bringing in $80 million in a sixth round from ESPN, immediately earmarked for acquisitions. Even before that, however, it had absorbed a number of smaller entities, including HY-TEK Sports Software (specializing in swimming, track and field applications) in July 2008. Two other recent purchases, Automated Licensing Systems and Outdoor Central, are used by many state governments to process hunting and fishing licenses.

To date, Active has raised $275 million in venture backing from ABS Ventures, Austin Ventures, Canaan Partners, Charles River Ventures, Dominion Ventures, ESPN, Kettle Partners, Enterprise Partners, North Bridge Venture Partners and Ticketmaster. Incidentally, ReserveAmerica was acquired by IAC’s Ticketmaster/Citysearch division for $22 million in 2001.