What Is a Subnet Mask? (Explained Simply)

If an IP address identifies your device on a network, the subnet mask tells the device:

“Which part of this IP address is the network, and which part is the device?”

This is a high-yield concept for the CompTIA A+ exam and real troubleshooting.

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What Does a Subnet Mask Do?

A subnet mask is a number that works with an IP address to determine:

• Which devices are on the same local network
• Whether traffic should go directly to another device OR to the default gateway (router)

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Most Common Subnet Mask (Home Networks)

The most common subnet mask you’ll see is:

255.255.255.0

Example IP configuration:

IP Address:   192.168.1.50
Subnet Mask:  255.255.255.0
Gateway:      192.168.1.1

On a /24 network (255.255.255.0), devices on the same local network usually share the first three numbers:

192.168.1.X

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How Subnet Masks Help Your PC Decide Where to Send Traffic

If your computer wants to talk to another device:

• If the device is on the same subnet, it sends traffic directly on the LAN.
• If the device is on a different subnet, it sends traffic to the default gateway (router).

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How to View Your Subnet Mask in Windows

Open Command Prompt and run:

ipconfig

Or for more detail:

ipconfig /all

Look for the line labeled Subnet Mask.

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Common Subnet Masks to Recognize (A+ Level)

• 255.255.255.0 → very common home/office LANs
• 255.255.0.0 → larger internal networks
• 255.0.0.0 → very large networks (less common in small environments)

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Real-World Troubleshooting Example

A student laptop can’t print to a network printer, but internet works fine.

You check the IP settings and find:

IP Address:   192.168.1.55
Subnet Mask:  255.255.0.0

But all other devices are using:

Subnet Mask:  255.255.255.0

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Subnet Mask vs Default Gateway

• Subnet mask tells your device what is “local.”
• Default gateway is where traffic goes when it’s not local (router).

If either is wrong, you can get:

• No internet
• Can’t reach printers/servers
• Intermittent network weirdness

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Related Networking Articles

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Recommended A+ Study Tools

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Final Summary

• A subnet mask defines what your device considers “local.”
• The most common mask is 255.255.255.0.
• Wrong subnet masks can cause confusing connectivity problems.
• Subnet mask works alongside the default gateway and DHCP.