What Is a Subnet Mask? 255.255.255.0 Explained
A subnet mask decides which addresses count as local. Here is 255.255.255.0 explained, common masks, CIDR and how to find yours.
A subnet mask tells your device which part of an IP address identifies the network and which part identifies the device. With it, your phone knows whether another address is in your home or out on the internet.
A subnet mask tells your device which part of an IP address is the network and which part is the device. At home it is almost always 255.255.255.0.
At home the mask is almost always 255.255.255.0. This guide explains what that means, the other common masks, CIDR notation, and how to find yours.
Subnet mask meaning in plain words
An IPv4 address has four numbers, such as 192.168.1.25. The mask has four numbers too. Where the mask says 255, that part of the address is the network. Where it says 0, that part is the device.
So with 192.168.1.25 and 255.255.255.0, the network is 192.168.1 and the device is .25. Any address starting with 192.168.1 is local. Anything else goes to the default gateway.
Common subnet masks
| Subnet mask | CIDR | Usable addresses | Typical use |
|---|---|---|---|
| 255.255.255.0 | /24 | 254 | Almost every home network |
| 255.255.255.128 | /25 | 126 | Splitting a /24 in two |
| 255.255.254.0 | /23 | 510 | Larger homes and offices |
| 255.255.0.0 | /16 | 65,534 | Big private networks |
| 255.0.0.0 | /8 | About 16.7 million | The whole 10.0.0.0 range |
| 255.255.255.252 | /30 | 2 | Point-to-point links |
Try any of these in our Subnet Calculator to see the network, broadcast and host range.
What is CIDR notation?
CIDR writes the mask as a slash and a number, counting how many bits belong to the network. 255.255.255.0 has 24 network bits, so it is written /24, as in 192.168.1.0/24.
CIDR is shorter and is used in router settings, firewalls and cloud networking. Each step up halves the number of addresses: /24 has 256, /25 has 128, /26 has 64.
How to find your subnet mask
- Windows: run
ipconfigand read Subnet Mask. - Mac: System Settings, Network, your connection, Details, TCP/IP, Subnet Mask.
- Linux: run
ip addrand read the number after the slash, for example /24. - Android and iPhone: WiFi settings, tap your network, and look for Subnet Mask in the details.
- Router: log in and open LAN settings.
Subnet mask problems
- Wrong mask on a static IP: devices cannot see each other or the router. Use the same mask as the router, usually 255.255.255.0.
- Two networks with the same range: a VPN or second router on 192.168.1.x can clash with your home network. Change one of them to another subnet.
- Need more than 254 devices: change the router LAN mask to 255.255.254.0 and widen the DHCP range.
A quick worked example
Say your laptop shows IP 192.168.1.57 with subnet mask 255.255.255.0. Here’s what that tells you, step by step.
- The first three numbers, 192.168.1, are the network part. Every device on your home network shares them.
- The last number, 57, is your laptop’s own spot on that network.
- Usable addresses run from 192.168.1.1 to 192.168.1.254, which is 254 devices.
- 192.168.1.0 is the network address and 192.168.1.255 is the broadcast address. Neither goes to a single device.
So if your phone has 192.168.1.80, it’s on the same network as the laptop and they can talk directly. If a printer has 192.168.0.20, it’s on a different network, and the laptop won’t see it without a router in between. That mismatch is behind a lot of “my printer disappeared” problems.
Want to try other combinations? Our Subnet Calculator shows the range, broadcast address and host count for any IP and mask.
Common subnet masks at a glance
You’ll rarely need anything other than the first one at home, but these show up on business networks and in router settings:
- 255.255.255.0 (/24): 254 devices. Standard for almost every home router.
- 255.255.254.0 (/23): 510 devices. Used when a home or office outgrows 254.
- 255.255.0.0 (/16): 65,534 devices. Large company networks.
- 255.255.255.252 (/30): 2 devices. Links between two routers.
- 255.0.0.0 (/8): over 16 million devices. The full 10.x.x.x private range.
Why your network uses a subnet mask at all
Every time a device sends data, it asks a simple question: is this address on my own network, or somewhere else? The subnet mask is how it answers.
If the destination is local, like your printer at 192.168.1.30, the device sends the data straight there. If it’s outside, like a website, the device hands it to the default gateway (your router) and lets the router pass it on.
Get the mask wrong and a device can end up trying to deliver internet traffic locally, or ignoring a printer sitting right next to it.
That’s also why you almost never need to touch it. Your router hands out the right mask with every IP address through DHCP. You only set it by hand when you give a device a static IP, and then it should match what the router uses.
How to find your subnet mask on any device
- Windows: press Windows + R, type cmd, then run
ipconfig. Look for Subnet Mask under your WiFi or Ethernet adapter. - Mac: System Settings, Network, your connection, Details, then TCP/IP. It’s listed as Subnet Mask.
- iPhone: Settings, WiFi, tap the (i) next to your network. Subnet Mask is under IPv4 Address.
- Android: Settings, Network, WiFi, tap the network, then Advanced. Some phones show a prefix length like 24 instead.
- Router: log in and open the LAN settings page. The mask there is the one every device should match.
If one device shows something different from the others, it was probably given a static IP by hand. Switching it back to automatic is the quickest fix.
