At 5 o'clock tonight, as soon as the long-distance rates go down, The Network Nation will be on the air.

Tonight, like every other night, Americans of all ages in every corner of the country will connect a personal computer to a telephone and talk to one another through a vast and varied network of electronic "bulletin boards" -- a whole new form of mass communication that is cheaper, more accessible and less regulated than almost any other national medium.

Until recently, these electronic networks were largely the preserve of youthful computer "hackers," like the seven New Jersey teen-agers who were arrested this week and charged with using a bulletin board to plan computer crimes.

But in the past year or so, the bulletin board system, or "BBS," has entered the American mainstream. There are now thousands of bulletin boards serving the special interests of pilots, parsons, professors, planners, politicians and a potpourri of other professions.

Fundamentalists exchange electronic prayers over the "Computers for Christ" system. Wine snobs debate their favorites on "On-Line Wine." Pornography fans tap into "Micro-Smut." Princeton alumni exchange high-tech harrumphs about coeducation and other modern invasions of their alma mater via the "Electronic PAW." Liberal activists plan their marches on "Voice of Peace" while right-wing extremists post hate messages on the "Aryan Nations Liberty Net."

There are bulletin boards providing legal, financial and psychiatric advice. Most big cities have at least one "Dial-Your-Match" dating service bulletin board, and computerized romance seems to be flourishing. Hundreds of computer owners phoned in to attend, electronically, the on-line marriage of George and Debbie Stickles, a Grand Prairie, Tex., couple who met and courted on a bulletin board while living hundreds of miles apart.

Political figures and opinion leaders are eager to board this fast-moving bandwagon.

"This is an enormously important information resource, and it's available for anyone," said Ralph Nader, who has pushed for creation of a national network of consumer bulletin boards.

"Not many people can buy a big national magazine or a TV network," Nader said. "But anybody with a computer and a $100 software program can start one of these systems. It's the lowest barrier to entry of any mass communication medium."

David Hughes, a retired Army officer in Colorado Springs, Colo., who is sometimes called the president of The Network Nation because of his ubiquitous presence on major bulletin boards, argues that the computer network represents a new form of town meeting suited to an entire country.

"Personal computers will be used as naturally for communication and free assembly as the telephone and meeting places are now," he said.

Hughes emphasizes the political use of the medium. "Tom Paine would have first published his 'Common Sense' on a local computer bulletin board," he suggested.

There are no solid figures on the number of Americans who now "access" computer bulletin boards. Computer industry analysts say somewhat less than 10 percent of the nation's 85 million households have personal computers. A large majority of those home systems include a "modem," the gadget that marries a computer to a telephone and permits access to bulletin boards.

That would suggest that several million Americans can hook up to bulletin boards from their homes, in addition to those who have access to a computer system at work.

It is equally difficult to state accurately how many different bulletin board systems there are. Industry estimates range upward from 2,000, but the number seems to be growing weekly, because computer owners can start a system with a minimal additional investment and operate it through their home telephones.

There are three basic types of computer communication systems.

The most numerous are the free bulletin board systems all over the country -- such as "Computers for Christ" -- dispensing conversation and information to anyone who chooses to call. There generally is no fee for these services, although callers have to pay any long-distance charges run up while they are connected.

There are also a few dozen commercial bulletin boards -- the largest are CompuServe, the Source and the Dow-Jones News/Retrieval System -- which provide services ranging from news reports and stock quotations to gardening tips and airline schedules. These electronic magazines generally charge a fee ranging upward from $6 per hour.

Finally, there are hundreds of private computer bulletin boards used by businesses, schools, government agencies and professional organizations to keep in touch with a national roster of members or employes.

This news story, for example, was sent to The Washington Post's composing room over a private Post network, and was then redistributed to other papers around the world over a separate private system run by the Los Angeles Times-Washington Post News Service.

One of the more common forms of computer crime -- including the crimes charged against the New Jersey teen-agers -- involves a hacker who tries to penetrate one of these private networks.

Normally, a private network has its own telephone number and one or more passwords that a caller must type in to gain access to the system. Most private bulletin boards change these numbers and passwords regularly to frustrate invasions from hackers.

But last year a group of hackers obtained a phone number and password for a credit-rating firm owned by TRW Inc. With that information, the hackers were able to call up and obtain the credit card numbers and financial records of any of the millions of people listed by the firm.

Burt Mazelow, from Garden Grove, Calif., whose credit card numbers became public in the hacker community, has sued TRW for failure to provide adequate security of its records.

Last fall some bulletin boards frequently called by young hackers listed a telephone number for "Arpanet," a Defense Department research computer system. The necessary passwords were not listed, however, and the Pentagon said there was no evidence of unauthorized calles to Arpanet.

Mainstream bulletin board-system operators condemn these hackers and maintain that they represent a minute portion of The Network Nation. Hughes, the Colorado Springs computer buff, dismisses the hackers as a small group of "keyboard vagrants passing through our electronic neighborhood."

And hackers now emphasize the need to stay legitimate. Even such youth-oriented bulletin boards as "Phreakenstein's Lair," where teen-agers heatedly debate rock music, the Dungeons and Dragons game and new computer equipment, flash a warning on the screen to all callers: "Anyone leaving any message . . . dealing with breaking into computers, etc., will have their password ZAPPED!!!!!"