NEW YORK - Happy birthday?, Internet. By some accounts, Wednesday marked the 20th anniversary of the online medium.

It was on Jan. 1, 1983, that the 400 or so computers hooked to what was then called ARPANET had to switch to a communications protocol called TCP/IP, said Vint Cerf, the protocol's co-inventor.

It was TCP/IP that allowed multiple networks to coexist and permitted applications like the World Wide Web to develop and thrive. In other words, it made the Internet what it is today.

"This is a major milestone," Cerf said. "I consider the January 1983 date to be the real rollout of (the) Internet."

Some, however, consider the Internet's age to be a more mature 33.

On Sept. 2, 1969, two computers at the University of California, Los Angeles, linked by a 15-foot cable, sent data back and forth, showing that the Internet could work.

Sure, the protocol didn't permit non-ARPANET computers to join in, the way America Online and private corporate networks can today. But it affirmed packet switching, the idea that data could be chopped into small packets and reassembled at the destination, giving the Internet its versatility.