What Is a Subnet Mask? 255.255.255.0 Explained

Next to every IP address in your network settings sits a strange companion — usually 255.255.255.0. That's the subnet mask, and while it looks cryptic, its job is simple: it tells your device which addresses are local (on your own network) and which are remote (reachable only through the router).

The core idea: splitting an address in two

Every IP address has two parts: a network portion (which network you're on) and a host portion (which device you are on that network). The subnet mask marks the dividing line. Wherever the mask has 255, that part of the address identifies the network; wherever it has 0, that part identifies the device.

With IP 192.168.1.37 and mask 255.255.255.0:

• Network: 192.168.1 — shared by every device in your home
• Host: .37 — this specific device

Why your device cares

When your laptop wants to talk to 192.168.1.50, it checks the mask, sees the network part matches its own, and sends the data directly across the local network. When it wants to reach 142.250.80.46 (a Google server), the network part doesn't match — so it hands the packet to the default gateway (your router) to forward to the internet. That one decision — local or not — is the subnet mask's entire purpose.

CIDR notation: the modern shorthand

You'll often see masks written as a slash and a number, like 192.168.1.0/24. The number counts how many bits belong to the network:

When subnet masks cause problems

A wrong mask is a classic source of "I can reach the internet but not my printer" problems. If two devices on the same physical network have mismatched masks, one may think the other is remote and route local traffic out through the router, where it dies. If local devices can't see each other, comparing subnet masks is one of the first things to check — along with IP conflicts.

Frequently asked questions