What is a local network?

A local network, or LAN, is the set of devices connected to the same router, over Wi-Fi or by cable. Your laptop, your phone, a games console and a printer plugged into the same box at home are all on one local network.

What makes it useful is that these devices can talk to each other directly. Data going between them passes through the router and stops there. It never touches your internet connection, which is why local transfers aren't limited by your broadband speed and keep working when the internet is down.

Every device on your network gets a private IP address, handed out automatically by the router. They come from ranges reserved for private use, so you'll start recognising them once you know what to look for:

These addresses aren't unique in the world. Millions of homes have a device at 192.168.1.5. They only have to be unique inside your own network, and they can't be routed across the internet. A server can't dial 192.168.1.5 and reach you, because it has no idea whose it would be.

Your router holds the one address that is globally unique: your public IP, issued by your internet provider. It translates between the two, a job called NAT, so lots of private devices can share a single public address.

Two devices are on the same network when they share a subnet. Roughly, that means their addresses match for the part the subnet mask covers. With the usual home setup of 192.168.1.x and a 255.255.255.0 mask, anything from 192.168.1.1 to 192.168.1.254 is local to everything else in that range.

This matters more than it sounds. Most device discovery works by sending a message to everything on the local subnet at once, and those messages don't cross from one subnet into another. Two devices can be in the same room, on the same router, and still fail to find each other because they've been put on different subnets. That's exactly what a guest network usually does.