To play Jazz Jackrabbit online you'll need several things.
Jazz Jackrabbit 2 1.21 to 1.23 patch - If you have the normal version of Jazz Jackabbit 2 (not any of the expansion packs, The Secret Files or Christmas Chronicles), you will need to install the 1.23 patch, which updates your version of Jazz Jackrabbit 2 to 1.23. You won't be able to play online without it. The patch updates the files in your Jazz Jackrabbit 2 directory. It will also fix several bugs in the game and add several features. This patch was released by the game's developer, Epic Games.
JJ2+ (beta) - A large JJ2 patch created by BlurredD. Has many bugfixes and new features, such as the new Team Battle and Last Rabbit Standing gametypes and spectator mode. A lot of online servers use and sometimes even require this patch.
Note: if you get error messages about your game version when running the patch, try if the following forum topic clears things up: Versions of Jazz Jackrabbit 2, and why patches don't work.
Multiplayer launchers open up extra multiplayer features in Jazz Jackrabbit 2, and (usually) ease the process of starting servers.
These programs offer an easy way to customize your in-game name and your character's colours
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