Geyser
A translation layer that lets Minecraft Bedrock Edition players (console, mobile, Windows) join a Java Edition server.
Geyser is an open-source project by GeyserMC that makes crossplay between Minecraft Java Edition and Bedrock Edition possible. Bedrock players on consoles, phones and Windows connect to Geyser, and Geyser translates their connection into the Java protocol. To the Java server, they look like normal Java players.
Where it runs
Geyser can run as a plugin or mod on the server itself (for example on Paper, Fabric or NeoForge), on a proxy such as Velocity, or as a standalone program. On a network, installing it on the proxy is usually the cleanest option.
Ports
Bedrock uses UDP, not TCP, and its default port is 19132. Geyser therefore needs its own UDP port open in addition to the Java port. Bedrock clients also do not support SRV records, so they must enter the address and port directly. See port and SRV record.
Accounts
On its own, Geyser still requires a Java account: Bedrock players have to link or log in with one. To let Bedrock players join with only their Microsoft/Xbox account, add Floodgate.
Why it matters for server owners
Crossplay lets friends on consoles and phones play together with Java players on the same world. The trade-off is that translation is not perfect. Some Java features have no Bedrock equivalent, and combat, some block and item behaviour, and custom content like resource packs may look or feel different. Plugins with custom GUIs or items may need extra work.
Practical tip
Install Geyser and Floodgate together, and keep both updated whenever Bedrock releases a new version; Bedrock clients update automatically and older Geyser builds then stop accepting them. If Java players need to join from different versions too, combine it with ViaVersion.