What is a subnet mask, and how to read yours
Short answer: a subnet mask marks off which part of your IP address identifies the network and which part identifies the individual device on it. On most home routers that mask is 255.255.255.0, which means the first three numbers of your IP address are fixed and only the last one changes per device.
What the mask actually does
Every device on your WiFi gets a local IP address like 192.168.1.42. That number alone doesn’t say which part is “the network” and which part is “this specific device.” The subnet mask answers that. Lined up against the IP address, it tells your router: everything up to here is the network, everything after is the device.
That split matters because it’s how your router decides whether traffic needs to go anywhere. If a device is asking for something inside the same network part, the router hands the packet straight to the recipient. If it’s asking for something outside that range, the router sends it out to the internet instead.
Reading 255.255.255.0
A subnet mask is written as four numbers, same format as an IP address. Each 255 means “this whole number is part of the network,” and a 0 means “this whole number is free for devices to use.”
With a mask of 255.255.255.0 and an IP address of 192.168.1.42:
192.168.1is the network part, shared by every device on your WiFi.42is this device’s own slot within that network.
Change the router and connect a laptop, and it might get 192.168.1.15 instead. Same network, different last number. If two devices somehow ended up with the same full address, they’d conflict, which is why your router hands these out automatically rather than leaving it to chance.
CIDR notation: the /24 shorthand
You’ll often see the same information written as /24 instead of 255.255.255.0. That’s CIDR notation, and the number is just a count of how many bits (out of 32 total in an IP address) belong to the network part. /24 means 24 of the 32 bits are fixed as the network, leaving 8 bits free for devices. Eight bits gives 256 possible combinations, so a /24 network has 256 addresses total. Two of those are reserved (one for the network itself, one for broadcasting to every device at once), which leaves 254 usable addresses for actual devices.
You’ll see /24 in your router’s admin page, in gateway settings, and in any networking tool that expects CIDR input, including the subnet calculator if you want to work out the range for a mask you’re not sure about.
Why home networks use /24
254 usable addresses covers essentially any household: phones, laptops, smart plugs, a TV, a doorbell camera, guest devices. Most consumer routers ship with 255.255.255.0 as the default for exactly that reason, and most people never have a reason to touch it.
When you’d change it
The only common reason to move off the default is deliberately splitting your network into separate pieces, for example putting IoT devices on their own segment so a compromised smart plug can’t see your laptop. That’s a real setup some routers support natively through guest networks or VLANs, and it’s worth doing if you’re security-conscious about a large number of smart-home devices. Outside of that, a different mask usually means something is misconfigured rather than something you set on purpose.
Finding your own
Open your router’s admin page and look at the LAN or network status section. The subnet mask sits right next to the router’s own IP address, almost always shown as either 255.255.255.0 or /24. If your router’s admin page is hard to reach in the first place, finding your gateway address is the step before this one.
Frequently asked questions
What does 255.255.255.0 mean?
It's a /24 mask: the first three numbers of your IP address (the network part) are fixed for every device on your WiFi, and only the last number changes per device. That gives room for 254 usable device addresses on the network.
Do I need to change my subnet mask?
Almost never on a home network. The default your router ships with already fits typical household device counts. You'd only touch it to split a network into separate segments, like isolating IoT devices from your main devices on purpose.
What's the difference between a subnet mask and CIDR notation?
They're the same information written two ways. 255.255.255.0 and /24 both mean the first 24 bits of the address are the network part. CIDR notation is just the shorter form you'll see in router settings and technical docs.