What a subnet mask does and why you need one
A subnet mask is a number that tells your router and devices which part of an IP address identifies your network and which part identifies individual devices on that network. When you set up a home network, your router assigns both an IP address and a subnet mask to each device. The subnet mask works like a filter: it shows devices how to talk to each other on your local network versus how to send traffic out to the internet.
You do not have to calculate a subnet mask yourself in most home setups — your router does it automatically. But understanding how they work helps you troubleshoot connection problems, recognize when a device is on the wrong network, and make sense of what your router is actually doing when you look at its settings page.
Key Takeaways
- A subnet mask separates the network portion of an IP address from the device portion, using the same four-number format as the IP address itself.
- The most common home subnet mask is 255.255.255.0, which means the first three numbers of the IP address identify your network and the last number identifies individual devices.
- You can calculate a subnet mask by converting the network size to binary, but most home routers assign one automatically based on the IP address range you choose.
- CIDR notation (like /24) is shorthand for subnet masks and appears in router settings and network documentation.
- Mismatched subnet masks are a common reason devices cannot see each other on a home network, even when they have the correct IP addresses.
The standard subnet mask for home networks
Most home routers use a subnet mask of 255.255.255.0. This mask divides a typical home IP address range (like 192.168.1.0 to 192.168.1.255) so that the first three numbers stay the same across your whole network, and only the last number changes from device to device.
If your router assigns you the IP address 192.168.1.50 with the subnet mask 255.255.255.0, that means your network is 192.168.1.0, and your device is number 50 on that network. Another device on the same network might be 192.168.1.75. Both devices recognize each other as local because the first three numbers match and the subnet mask says "the first three numbers are the network part."
You will see this same mask written as /24 in some places — that is CIDR notation, which counts how many bits of the address belong to the network. With 255.255.255.0, that is 24 bits, so /24 and 255.255.255.0 mean the same thing.
How to read an IP address and its subnet mask together
An IP address is four numbers from 0 to 255, separated by dots. A subnet mask is also four numbers from 0 to 255, separated by dots. To see which part of the IP address is the network and which part is the device, line them up and look at where the mask changes from 255 to 0.
| IP Address | 192.168.1.50 |
|---|---|
| Subnet Mask | 255.255.255.0 |
| Network Portion | 192.168.1 |
| Device Portion | 50 |
The 255 values in the mask mean "this part of the IP address is the network." The 0 values mean "this part is the device address." So in the example above, every device on your network will have 192.168.1 as the first three numbers, and the last number will be different for each device (1 through 254 — 0 and 255 are reserved).
If a device on your network has the IP address 192.168.2.50 instead, the subnet mask 255.255.255.0 tells your devices that this address is on a different network, even though it is only one number different. That device will not be able to talk directly to your other devices.
Calculating a subnet mask from an IP address range
If you need to set up a subnet mask manually — which is rare in home networks but sometimes happens when you are configuring a guest network or advanced router settings — you start by deciding how many devices you need to support on that network.
A subnet mask of 255.255.255.0 supports up to 254 devices (the numbers 1 through 254 in the last position). If you need fewer devices, you can use a larger mask like 255.255.255.128, which supports 126 devices. If you need more, you would use a smaller mask like 255.255.0.0, which supports about 65,000 devices.
The calculation works by converting to binary. Each number in the mask is 8 bits. 255 in decimal is 11111111 in binary (all bits on). 0 in decimal is 00000000 in binary (all bits off). 128 is 10000000 in binary (first bit on, rest off). When you add up the "on" bits across all four numbers, you get the CIDR notation: 255.255.255.0 is 8+8+8+0 = 24 bits, so /24.
For a home network, you almost never need to do this calculation. Your router chooses a subnet mask automatically when you select the IP address range during setup. But if you are reading documentation or troubleshooting a network that is not working, understanding this relationship helps you spot when a device has the wrong mask.
Common subnet masks and what they mean
| Subnet Mask | CIDR Notation | Devices Supported | Common Use |
|---|---|---|---|
| 255.255.255.0 | /24 | 254 | Standard home network |
| 255.255.255.128 | /25 | 126 | Small home network or guest network |
| 255.255.255.192 | /26 | 62 | Very small network or IoT devices only |
| 255.255.0.0 | /16 | 65,534 | Large office or advanced setup |
The /24 mask is the default for most home routers because it balances simplicity with enough room for all the devices a typical household needs. If your router is set to 192.168.1.0 with a /24 mask, you can have devices numbered 192.168.1.1 through 192.168.1.254.
When you look at your router's settings page, you will usually see the subnet mask listed next to the IP address. If you see a mismatch — for example, one device showing 255.255.255.0 and another showing 255.255.0.0 — that is why they cannot communicate, even if the first three numbers of their IP addresses look similar.
Why subnet masks matter when devices cannot find each other
One of the most common home network problems is a device that has the right IP address but the wrong subnet mask. This happens most often with devices you manually configure instead of letting the router assign settings automatically. The device can connect to the internet through the router, but it cannot see other devices on your local network.
If you have a printer, security camera, or computer that is not showing up on your network, check two things: first, that it has an IP address in the same range as your other devices (like 192.168.1.x if your network is 192.168.1.0). Second, that it has the same subnet mask as your router (usually 255.255.255.0). If the mask is different, the device thinks it is on a different network and will not try to talk to your other devices directly.
Your router's settings page will show you the subnet mask it is using. Most routers have this information under a section called "LAN Settings" or "Network Settings." Write down the mask you see there, and make sure any device you configure manually uses the same one.
When you might need to change a subnet mask
In a standard home network, you never need to change the subnet mask. Your router picks one when you set it up, and it works for any reasonable number of home devices.
The only time you might adjust it is if you are setting up a guest network that you want to keep separate from your main network, or if you are configuring a device on a network that is not your home network (like a work network or a network at a school). In those cases, you would use the same subnet mask as the network you are joining, not create a new one.
If a technician or documentation tells you to change your subnet mask, ask them why. The answer should be specific to your situation — for example, "your office network uses /25 instead of /24" — not a general recommendation. Changing the mask without a reason will break communication between devices that were working before.
Frequently Asked Questions
What is the difference between a subnet mask and an IP address?
An IP address identifies a specific device on a network. A subnet mask tells devices which part of the IP address is the network and which part is the device. You need both for devices to find each other.
Can I use a different subnet mask on different devices?
No. All devices on the same network must use the same subnet mask, or they will not recognize each other as being on the same network. If you have devices with different masks, they cannot communicate directly.
Why do I see /24 instead of 255.255.255.0 in my router settings?
Both mean the same thing. /24 is CIDR notation, which counts the number of bits in the network portion of the address. 255.255.255.0 is the older decimal notation. Some routers show one, some show both.
What happens if I set the subnet mask to 255.255.0.0 on a home network?
Your devices will still work, but you will have created a much larger network than you need. This is not harmful in a home setting, but it is unnecessary and makes the network harder to manage if you ever need to troubleshoot.
Do I need to know how to calculate a subnet mask to set up a home network?
No. Your router calculates and assigns subnet masks automatically. Understanding how they work helps you troubleshoot, but you do not need to calculate one yourself for a standard home setup.