ExampleMessageHandler.java

package com.examplemod.network;

import io.netty.buffer.ByteBuf;

import net.minecraft.world.World;

import cpw.mods.fml.common.network.simpleimpl.IMessage;
import cpw.mods.fml.common.network.simpleimpl.IMessageHandler;
import cpw.mods.fml.common.network.simpleimpl.MessageContext;

public class ExampleMessageHandler implements IMessageHandler<ExampleMessageHandler.ExampleMessage, IMessage> {

    public ExampleMessageHandler() {} // needed for FML to be able to create instances of this handler

    @Override
    public IMessage onMessage(ExampleMessage message, MessageContext context)
    {
        // this example is for server handler
        if(!context.side.isServer())
            throw new IllegalStateException("received ExampleMessage " + message + "on client side!");

        World world = context.getServerHandler().playerEntity.worldObj;
        // World world = Minecraft.getMinecraft().theWorld; // on the client

        world.markBlockForUpdate(message.x, message.y, message.z); // or whatever

        return null; // no reply
    }

    public static class ExampleMessage implements IMessage {
        public int x, y, z;

        public ExampleMessage() {} // needed for Netty to create instances to read into

        public ExampleMessage(int x, int y, int z)
        {
            this.x = x;
            this.y = y;
            this.z = z;
        }

        @Override
        public void fromBytes(ByteBuf buf)
        {
            x = buf.readInt();
            y = buf.readInt();
            z = buf.readInt();
        }

        @Override
        public void toBytes(ByteBuf buf)
        {
            buf.writeInt(x);
            buf.writeInt(y);
            buf.writeInt(z);
        }
    }
}

somewhere in Mod.java

// somewhere convenient

public static SimpleNetworkWrapper netHandler = NetworkRegistry.INSTANCE.newSimpleChannel(modId + "|B"); //doesn't have to have this suffix, just an example

// call sometime before using network stuff

// 0 should be unique per message - they're dispatched that way
// side is the receiving side - the side where onMessage will be called
netHandler.registerMessage(ExampleMessageHandler.class, ExampleMessageHandler.ExampleMessage.class, 0, Side.SERVER);

// to send a packet to server:
netHandler.sendToServer(message);

// look inside SimpleNetworkWrapper class for additional send methods