ComputerCraft

Computer network via modem

Connect multiple computers wirelessly using the rednet API. Send commands, receive status updates, and remotely control machines across your base.

Category: ComputerCraft Difficulty: Intermediate Time: ~20 min

What you will end up with

Two computers communicating wirelessly: a sender that broadcasts commands, and a receiver that listens and acts on them — for example, toggling a redstone output or printing a status message.

What you need

Two Computers (from CC:Tweaked)
Two Wireless Modems — one per computer, placed on any side
Both computers within modem range (vanilla wireless modem: 64 blocks; Ender Modem: unlimited)

Each computer has a unique numeric ID. You can see it at the top of the terminal or with the command id. You will need the receiver's ID for the sender script.

How rednet works

rednet is a high-level messaging API built on top of the raw modem API. You open it on the side where the modem is attached, then use rednet.send() to send to a specific computer ID or rednet.broadcast() to send to all computers in range.

Step 1 — Set up the receiver

On Computer A (the receiver), attach a wireless modem to the top and create this script:

-- receiver.lua
-- Listens for commands and executes them

rednet.open("top")
print("Receiver ready. ID: " .. os.getComputerID())

while true do
  local senderId, message = rednet.receive()
  print("From " .. senderId .. ": " .. tostring(message))

  if message == "activate" then
    redstone.setOutput("right", true)
    os.sleep(1)
    redstone.setOutput("right", false)
    rednet.send(senderId, "done")
  elseif message == "ping" then
    rednet.send(senderId, "pong")
  end
end

Run it:

receiver

Note the ID printed on screen — you need it for the sender.

Step 2 — Set up the sender

On Computer B (the sender), attach a wireless modem to the top and create this script. Replace TARGET_ID with the ID you noted from the receiver.

-- sender.lua
-- Sends a command to the receiver computer

local TARGET_ID = 5  -- replace with your receiver's ID

rednet.open("top")

print("Sending 'activate' to computer " .. TARGET_ID)
rednet.send(TARGET_ID, "activate")

local _, reply = rednet.receive(5)  -- wait up to 5 seconds
if reply then
  print("Reply: " .. reply)
else
  print("No reply received.")
end

rednet.close("top")

Run it:

sender

The receiver should print the message and toggle its redstone output.

Broadcasting to all computers

If you want to send a message to every computer in range without specifying an ID, use rednet.broadcast():

rednet.open("top")
rednet.broadcast("hello everyone")
rednet.close("top")

Using protocols

For more complex networks, pass a protocol string to filter messages by type:

-- Send with protocol
rednet.send(TARGET_ID, "status_ok", "my_protocol")

-- Receive only messages matching this protocol
local id, msg = rednet.receive("my_protocol", 5)

Troubleshooting

rednet.open() fails — make sure the modem is on the correct side and is a wireless modem (not wired).
No messages received — check that both computers have their modems open and are within range. Ender Modems work at any distance.
Messages from the wrong computer — always check the senderId returned by rednet.receive() before acting on a message.

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