If you are talking about the original stand alone RoC and TFT, those used to connect to the original Battlenet. For a while, it was changed where if one tried to connect, it forced the game to upgrade to Reforged. However, I’ve seen a couple of reports more recently that trying to connect does nothing now.
Moral of the story: WC3’s old Battlenet seems to be shutdown. If you want to play online, you’ll have to use the Reforged client.
You cannot play the original versions of RoC and TFT online. As Levi said, historically the game would update to the current version, 2.0.1, also known as “Reforged,” if you tried, but now it appears even that doesn’t happen.
You can use your existing Reign of Chaos CD Key by registering it to your battle.net account in account management- after this, you can install the game by following these steps:
Open The Battle.net App.
At the login prompt, click the gear icon above the username/password fields and select “Continue without logging in”
Navigate to Warcraft III in the battle.net app and install it
Log in with your account and you will be able to play.