What Is a Subnet Mask?

by Priya Nakamura Updated Apr 12, 2026

A subnet mask determines which part of an IP address identifies the network and which part identifies individual devices. The most common home subnet mask is 255.255.255.0.

How Subnet Masks Work

A subnet mask like 255.255.255.0 tells devices that the first three numbers of the IP address identify the network (e.g., 192.168.1.x), and the last number identifies the specific device. This means up to 254 devices can exist on the network.

Common Subnet Masks

Home networks almost always use 255.255.255.0 (also written as /24). This allows 254 devices, which is more than enough for any home. Larger networks use different masks like 255.255.0.0 (/16).

Related Guides

Video Tutorial

For more information, see RFC 791 — Internet Protocol.

Pro Tip: Every device on the internet has an IP address, but thanks to NAT, hundreds of devices can share a single public IP.

Key Takeaways

Private IPs (192.168.x.x, 10.x.x.x) are used inside your home network
DNS translates domain names to IP addresses
DHCP automatically assigns IPs so you don't have to
Your default gateway is your router's IP address

Frequently Asked Questions

What is a subnet mask in simple terms?

A subnet mask tells devices which part of an IP address is the network address and which part identifies individual devices.

What subnet mask should I use?

For home networks, always use 255.255.255.0. This is the default and supports up to 254 devices.

About Priya Nakamura

Priya Nakamura is a telecommunications engineer and networking educator with a Master degree in Computer Networks and a background in ISP infrastructure design and management. Her experience spans both the technical architecture of broadband networks and the practical challenges home users face when configuring routers, managing wireless coverage, and understanding connectivity standards. At RouterHax, she covers WiFi standards and protocols, networking concepts, IP addressing, and network configuration guides.