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Choose the correct ZS Pack branch first
ZS Pack (Tensura) is a Zeefski modpack based on That Time I Got Reincarnated as a Slime. The project currently lists three Minecraft branches. They are separate installations, not interchangeable profiles: a 1.21.1 NeoForge server cannot accept a 1.19.2 Forge or 1.16.5 Forge client.
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| Minecraft branch | Loader | Client file listed | Matching server archive |
|---|---|---|---|
| 1.21.1 | NeoForge | ZS REINCARNATED-3.1.zip |
ZS_REINCARNATED_3.1-server-pack.zip |
| 1.19.2 | Forge | Not stated | ZS_REINCARNATED_SERVER_2.7.zip |
| 1.16.5 | Forge | Not stated | ZS_PACKserverpack.zip |
The latest listed client file is ZS REINCARNATED-3.1.zip, with the correspondingly named 1.21.1 server pack shown above. The project page reports 211,882 downloads in 2026 and a July 21, 2026 update. If you deliberately choose an older branch, download that branch’s server archive and install its matching Forge version instead of copying files between branches.
What you need before installing
A suitable machine
- A 64-bit operating system with enough free disk space for the server files, generated world, backups, and logs.
- A stable wired or wireless network connection. The host must stay powered on while people play.
- Administrator access to install Java, create a firewall rule, and, for internet play, configure the router’s port forwarding.
- Extra memory beyond the amount assigned to Minecraft so the operating system and other services remain responsive.
Java and loader requirements for 1.21.1
Use a 64-bit Java 21 runtime for the 1.21.1 NeoForge branch. NeoForge’s compatibility guidance states that Minecraft 1.20.5 and newer use Java 21. A 32-bit runtime, Java 17, or a different Java executable can cause an immediate launch or loader failure even when the pack files are correct.
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Verify the runtime from a terminal or Command Prompt with:
java -versionConfirm that the output identifies version 21 and a 64-bit VM. If several Java versions are installed, make sure the java command used in this shell points to the Java 21 installation before running the NeoForge installer or the server scripts.
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Use the pack’s files page to obtain ZS_REINCARNATED_3.1-server-pack.zip. A client zip contains launcher-side configuration and mods but is not automatically a runnable dedicated server. The separate server archive is assembled for server startup and is the correct starting point for a hosted world.
Install the 1.21.1 NeoForge server
1. Create a clean, dedicated folder
Create a directory used only by this server, such as C:MinecraftZS-Reincarnated on Windows or ~/minecraft/zs-reincarnated on Linux. Avoid extracting into a downloads folder or into an existing world with unrelated mods. A dedicated path makes backups, permissions, updates, and log collection much safer.
2. Extract the server pack
Extract every file from ZS_REINCARNATED_3.1-server-pack.zip directly into the new directory. Do not leave the files one level deeper inside a second automatically created folder if the startup scripts are then difficult to find. Keep the original zip separately so you can re-extract a clean copy if an update or configuration experiment goes wrong.
Look for the pack’s startup files and NeoForge-related files. Some server packs already contain the loader installation; others provide the files needed for you to install it. Do not run an installer until you have checked whether the archive already includes a working NeoForge server layout.
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3. Install NeoForge only when the pack does not already contain it
For the 1.21.1 branch, obtain the NeoForge installer that matches Minecraft 1.21.1 and place it in the server directory. From that directory, run:
java -jar neoforge-installer.jar --installServerThe installer must run with Java 21. If its filename includes a version suffix, substitute that exact filename in the command. Installing a loader for another Minecraft branch is not a harmless downgrade; it creates a version mismatch that prevents the pack from loading.
4. Start once to create the EULA file
Use the supplied script from the server directory:
- Unix-like systems:
./run.sh - Windows:
run.bat
The first launch normally stops after creating eula.txt. This is expected and is not a crash. Open that file, change:
eula=falseto:
eula=trueSaving that value records your agreement to operate the server under the Minecraft EULA. NeoForged’s server guidance describes the same sequence: install, start, accept the EULA, and start again.
5. Start the server again
Run ./run.sh or run.bat a second time. The server should continue loading the mod list and generate a world instead of stopping at the EULA check. Leave the terminal or console open while testing; it shows loader errors, missing dependencies, memory failures, and connection attempts that are not visible from the game launcher.
Set memory without making an unsafe assumption
NeoForge modpacks need more memory than an unmodded server, but there is no universal amount that guarantees a particular player count. A provider setup guide uses 6 GB, expressed as -Xmx6G, as its baseline for this pack. Treat that as provider guidance, not as a measured minimum or a promise of capacity.
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Set the maximum heap in user_jvm_args.txt, which is the configuration file used by the supplied startup scripts. For the cited baseline, the relevant argument is:
-Xmx6GDo not allocate all physical RAM to Java. The operating system, NeoForge, native libraries, file cache, and any other services also require memory. If the machine has only 6 GB total, assigning a 6 GB heap can make the host swap or become unresponsive. Change the allocation in small steps while watching startup time, garbage-collection pauses, and the server log.
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Actual demand varies with:
- Number of concurrent players and how quickly they move apart.
- View distance and simulation distance.
- New-chunk generation and exploration of previously unseen terrain.
- World size, entities, farms, machines, and other modded activity.
- Whether the machine is also running a desktop, voice chat, backups, or other servers.
When startup is slow or unstable, inspect user_jvm_args.txt, available system RAM, and the server logs before simply adding more heap. More heap cannot repair a wrong Java version, a mismatched loader, a broken mod, or a saturated CPU.
Configure the network so friends can connect
Local network play
For players on the same home network, give them the host computer’s local IP address and keep the server’s default port, 25565, unless you have a specific reason to change it. Allow the Java process or the server port through the host operating system’s firewall.
Internet play
Remote players normally need TCP port 25565 allowed in two places:
- Create an inbound firewall rule on the computer running the server for TCP 25565.
- Forward TCP 25565 on the router to the host computer’s local IP address. Reserve that local address with a DHCP reservation or static configuration so the rule does not point to the wrong machine after a reboot.
Players connect using your public address, optionally followed by a colon and a non-default port if you changed it. Never publish an address until the server is running and you have checked that the firewall and router rule target the correct host. A server that works locally but not over the internet usually has a port-forwarding, firewall, carrier-grade NAT, or incorrect-address problem rather than a modpack problem.
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Install the matching client for every player
Each player must install the exact client release represented by the server. For the current branch, that means ZS REINCARNATED-3.1.zip on Minecraft 1.21.1 with NeoForge. A launcher such as CurseForge, Prism, or another launcher that supports importing modpacks can manage the client instance.
Before connecting, compare these three values on both sides:
| Check | Server | Every client |
|---|---|---|
| Minecraft version | 1.21.1 | 1.21.1 |
| Loader | NeoForge | NeoForge |
| Pack release | ZS REINCARNATED-3.1 server pack | ZS REINCARNATED-3.1 client pack |
Do not add or remove mods casually. Even a small difference in a required library, configuration file, or loader build can produce a missing-mod, registry, or handshake error. If you intentionally customize the pack, keep a written list of changes and apply the same compatible changes to every client; for reliable troubleshooting, first reproduce the problem with an unmodified matching release.
Back up the world before changing anything
Stop the server cleanly before copying its world directory. Back up the world, configuration files, and the exact server-pack archive used to create it. Store at least one copy outside the live server directory. A backup made while region files are being written can be incomplete, so use the server console’s normal stop command and wait for the process to exit.
Make a new backup before changing Minecraft versions, loaders, pack releases, Java versions, or major configuration settings. Keep old backups rather than overwriting the only copy. A world created on 1.21.1 should not be opened by an older branch as a test; downgrade attempts can make the world unusable.
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The server exits immediately with a Java or loader error
- Run
java -versionin the same account and shell that starts the server. - Confirm the runtime is 64-bit Java 21 for Minecraft 1.21.1.
- Check that the NeoForge installer matches 1.21.1 and that you did not install a Forge loader from the 1.19.2 or 1.16.5 branch.
- Review the first error in the console or log; later stack traces are often consequences rather than the cause.
The first launch stops and no world appears
Open eula.txt and change eula=false to eula=true, save it in the server directory, and start the same script again. Editing a different copy of the file has no effect, so verify the path shown by the startup script.
Players receive a missing-mod or incompatible-version message
Compare the client and server filenames, Minecraft versions, and loaders. The 1.19.2 Forge archive ZS_REINCARNATED_SERVER_2.7.zip, the 1.16.5 Forge archive ZS_PACKserverpack.zip, and the 1.21.1 NeoForge archive ZS_REINCARNATED_3.1-server-pack.zip belong to different branches. Mixing any of them can look like a network failure even when port 25565 is open.
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- Confirm the server console has completed startup and is still running.
- Have the player use the correct public address and port, not the host’s private LAN address.
- Check the host firewall for an inbound TCP 25565 rule.
- Check the router’s port-forward target and confirm the host’s local IP has not changed.
- If local connections work but internet connections do not, investigate upstream NAT or an ISP that blocks inbound connections.
The server is laggy or takes a long time to start
Check CPU load, available RAM, heap settings in user_jvm_args.txt, view and simulation distances, and the server logs. Generate-heavy exploration can create temporary pauses; repeatedly increasing -Xmx6G is not a fix if the machine lacks spare physical memory or the bottleneck is CPU time. Test with fewer players and avoid generating a large unexplored area while diagnosing the issue.
Self-hosting versus managed hosting
Running the pack on your own computer gives you direct control, but a managed Minecraft host can move the always-on workload to a data center. The decision is practical rather than specific to one provider. Confirm that any service supports custom server-pack uploads and explicitly supports NeoForge 1.21.1 before paying for a plan.
| Consideration | Self-hosting | Managed hosting |
|---|---|---|
| Cost | No recurring hosting bill, but you supply the computer, electricity, storage, and internet connection. | Recurring fee; hardware and bandwidth are supplied by the provider. |
| Technical control | Full access to Java, files, loader, firewall, and operating system. | Control depends on the panel and plan; some settings may be restricted. |
| Setup and updates | You install Java, NeoForge, the pack, updates, and firewall rules. | Panel tools can simplify deployment, but you still must upload the correct server pack and coordinate client versions. |
| Backups and restore | You design and test your own backup process. | Some plans include automated backups; retention and restore limits vary and must be checked. |
| Latency | Usually best for players near your home connection; upload quality matters. | Choose a location near the majority of players, subject to the provider’s available regions. |
| CPU and RAM | Limited by your machine and other workloads. | Limited by the purchased plan and any shared-resource policy. |
| Custom uploads | Complete freedom to extract and modify the archive. | Verify that the control panel accepts this server pack rather than only one-click vanilla or curated installations. |
| NeoForge support | You can install the matching loader yourself. | Confirm explicit 1.21.1 NeoForge support, Java 21 selection, console access, and port management. |
Whichever model you choose, retain a local copy of the world and the exact pack release. A host’s backup feature is useful, but it should not be your only recovery path.
A repeatable launch checklist
- Selected one branch and recorded its Minecraft version, loader, and pack release.
- Downloaded the separately named server-pack archive.
- Extracted it into a clean, dedicated directory.
- Verified 64-bit Java 21 for the 1.21.1 branch.
- Installed the matching NeoForge loader only if it was not already present.
- Started once, changed
eula=falsetoeula=true, and started again. - Reviewed
user_jvm_args.txt; treated-Xmx6Gas a baseline, not a guarantee. - Allowed TCP 25565 through the host firewall and router when remote play is required.
- Installed the identical client release and NeoForge loader for every player.
- Backed up the world before upgrades or major configuration changes.
What a successful installation looks like
A successful setup has the 1.21.1 NeoForge server running from its dedicated directory, with the EULA accepted and the world loaded without unresolved mod or Java errors. Local players can connect using the host’s private address; remote players can connect through the public address after TCP 25565 is forwarded. Every client presents the same ZS REINCARNATED-3.1 pack and loader during the connection handshake. At that point, routine administration consists of monitoring logs and resource use, making tested backups, and repeating the version checks before every pack or loader update.
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