A subnet mask tells a device which part of an IP address describes the local network and which part identifies the individual device. You usually see it beside the IP address, gateway, and DNS settings.
Does this affect you?
Use this if you saw Subnet Mask, Netmask, or a slash value like /24 in Windows, Mac, iPhone, Android, or a router admin page. Most home users do not need to change it, but understanding it helps when troubleshooting printers, cameras, and static IP settings.
Find your subnet mask
It appears in the same place as your IP address.
On Windows:
- Open Command Prompt.
- Run ipconfig.
- Look for Subnet Mask under the active Wi-Fi or Ethernet adapter.
On Mac:
- Open System Settings > Network.
- Select your active Wi-Fi or Ethernet connection and click Details.
- Open TCP/IP and read Subnet Mask.
On iPhone or Android:
- Open Settings > Wi-Fi.
- Tap the information button or settings gear beside the connected network.
- Look in the IPv4 details for Subnet Mask or Netmask.
What 255.255.255.0 means
On most home networks, the subnet mask is 255.255.255.0. With an address such as 192.168.1.42, the 192.168.1 part identifies the local network and the 42 identifies that specific device. This setup supports up to 254 usable device addresses, far more than a normal home needs.
More control
Mismatched masks can break local devices
Two devices may both reach the internet but fail to find each other if one has the wrong subnet mask. That can make printers, smart TVs, file sharing, or cameras disappear. Switching the device back to automatic IP settings usually fixes it.
Slash notation means the same thing
A subnet can also be written as /24 instead of 255.255.255.0. The slash number counts the network bits. For home networks, /24 is the common shorthand you may see in router advanced settings.
Do not edit it casually
Unless you are setting up multiple subnets, advanced routing, or following a specific support instruction, leave subnet mask on automatic through DHCP. Manual changes often create more trouble than they solve.
Sources
- Cloudflare Learning Center – What Is a Subnet Mask? (2025)
- Microsoft Support – TCP/IP addressing and subnetting basics (2025)
- Netgear Support – Understanding IP addresses and subnet masks (2025)