ViaVersion

ViaVersion does one thing, but a crucial one: translate the network protocol between Minecraft versions. Thanks to that, a player on a newer version can connect to a server running an older version — and you don't have to rebuild the whole server for every Minecraft release.

It's the most common reason Czech servers stay on an older version. Players have auto-update enabled in their launcher, and without ViaVersion they wouldn't be able to connect to the server at all. With it, you can wait until the other plugins release for the new version, while players keep playing normally in the meantime.

ViaVersion is intended for incoming players on a newer version. For connections from even older clients, you need the ViaBackwards add-ons (and for very old versions, ViaRewind) — they are installed alongside it and extend the range of supported versions. For players on a phone, tablet, or console (Bedrock), another pair is used: Geyser and Floodgate. They complement ViaVersion, but they solve a different problem, so they are often used together.

In practice, you don't have to worry about the plugin. After a server update, it adapts itself, and in the console it only occasionally reminds you that a newer version is available. When a problem appears, the most common cause is a version mismatch between ViaVersion and its add-ons.

One thing can't be avoided: a player on a newer version will see an older world, but the opposite isn't always true. Blocks and items that the older version doesn't know will be displayed to them as a substitute. It's worth informing players about this in advance — for example, in the MOTD or on the website.

1. Find out which version the server is running (/version in the console or in game).
2. Download ViaVersion (modrinth.com/plugin/viaversion, Versions tab) — versions are released for a specific Minecraft range.
3. Upload the .jar to the plugins folder. If you use a proxy (Velocity, BungeeCord), ViaVersion belongs on the proxy, not on individual servers — otherwise double translation and errors occur.
4. Restart the server (for a proxy, restarting the proxy is enough).
5. Type /viaversion list — you will see which players connected from a different version than the server is running.
6. If players with older versions should also be able to connect, add ViaBackwards (and for very old versions ViaRewind). Install them in the same place as ViaVersion and in the same version.
7. If players from phones or consoles should be able to connect, add Geyser and Floodgate.
8. After every major Minecraft update, check that a newer ViaVersion has been released and upload it. Usually only this plugin changes; the server itself remains.
9. Verify functionality: have someone with a different version connect, and check that /viaversion list vyp lists it.
10. Write on the website or in the MOTD which versions the server supports — it will save you questions from players.

Keep ViaVersion and its add-ons (ViaBackwards, ViaRewind) in mutually compatible versions. When they get out of sync, players from other versions can't connect or crash — in the console this shows up as a protocol error.
A newer player on an older server won't see what the older version doesn't know: new blocks will be displayed as a replacement, and new items may not work. This is not a plugin bug and can't be worked around — it's a property of translation between versions.
On a proxy, install ViaVersion on the proxy, not on each server. Then, on game servers, leave only what the specific server needs.
After a Minecraft update, the usual procedure is: replace ViaVersion with a newer version, restart, and check /viaversion list. Server samotn; you don't need to update.
When the server stalls, it's usually not ViaVersion — it runs efficiently. Look at the number of entities, chunks, and the view-distance setting.
Combination for complete coverage: ViaVersion + ViaBackwards + ViaRewind for Java players of all versions, and Geyser + Floodgate for Bedrock players (phone, console).