What Is a Subnet Mask? Subnetting Explained Simply
SkillVeris Team
Cloud & Security Team
A subnet mask divides an IP address into a network portion and a host portion, telling a device which devices are on its local network.
In this guide, you'll learn:
- Subnetting is the practice of splitting a larger network into smaller sub-networks for better organization, security, and efficient address use.
- CIDR notation, like /24, is a shorthand way of expressing a subnet mask alongside an IP address.
- A device compares its own subnet with a destination address to decide whether to send data directly or through a router.
- Smaller subnets reduce broadcast traffic and can isolate parts of a network from each other for security purposes.
1What Is a Subnet Mask?
A subnet mask is a 32-bit value that pairs with an IP address to tell a device which portion of the address identifies the network and which portion identifies the specific host on that network.
Without this division, a device would have no way to know whether another address is on its own local network or reachable only through a router.
2Why Subnetting Exists
Subnetting is the practice of dividing a larger network into smaller, more manageable sub-networks, and it solves several practical problems at once.
Smaller subnets reduce the amount of broadcast traffic each device has to process, make address allocation more efficient, and let administrators isolate groups of devices from each other.
- Reduced broadcast traffic: broadcasts stay within a subnet instead of flooding an entire large network.
- Better organization: departments or device types can be grouped into their own subnets.
- Improved security: subnets can be isolated from each other with firewall rules between them.
- Efficient address use: subnets can be sized to roughly match how many devices actually need addresses.
3How to Read a Subnet Mask
A subnet mask is written in the same dotted-decimal format as an IP address, such as 255.255.255.0, where each 255 represents eight bits reserved for the network portion.
Reading a mask means identifying how many of the 32 bits are set to 1 for the network portion versus set to 0 for the host portion — the more bits reserved for the network, the fewer addresses remain for hosts.
- 255.255.255.0: the first 24 bits identify the network, leaving 8 bits (up to 256 addresses) for hosts.
- 255.255.0.0: the first 16 bits identify the network, leaving 16 bits (up to 65,536 addresses) for hosts.
- 255.255.255.128: the first 25 bits identify the network, leaving 7 bits (up to 128 addresses) for hosts.
4CIDR Notation: The Shorthand
CIDR notation writes the subnet mask as a slash followed by the number of network bits, such as 192.168.1.0/24, instead of spelling out the full dotted-decimal mask.
The number after the slash is exactly the count of bits set to 1 in the mask, so /24 means the same thing as 255.255.255.0.
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5How Devices Actually Use the Subnet Mask
When a device wants to send data, it compares the destination address against its own subnet, determined by applying its subnet mask, to decide whether the destination is local or remote.
If the destination falls within the same subnet, the device sends the data directly; if not, it forwards the data to its default gateway (a router) to be routed elsewhere.
6A Worked Example
Consider the network 192.168.1.0/24, which has 256 possible addresses, from 192.168.1.0 through 192.168.1.255.
Splitting it into two /25 subnets creates 192.168.1.0/25 (addresses .0 through .127) and 192.168.1.128/25 (addresses .128 through .255), each with 128 addresses instead of one shared pool of 256.
Why You'd Split It This Way
Splitting a /24 into two /25 subnets might separate two departments or floors of a building onto their own local segments, each with its own broadcast domain.
7Common Mistakes When Learning Subnetting
A frequent early mistake is forgetting that the first address in a subnet is reserved to identify the network itself and the last is reserved as a broadcast address, so neither is usable by a host.
Another common mistake is confusing the subnet mask with the default gateway — the mask defines the size of the network, while the gateway is the specific address a device sends off-subnet traffic to.
8Practicing Subnetting
Subnetting is a skill best built through repetition with real numbers rather than memorized rules, since the same binary logic applies to every example once it clicks.
Study Notes on Computer Networks covers subnetting with structured practice problems alongside the addressing and routing fundamentals it builds on.
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SkillVeris Team
Cloud & Security Team
Our cloud and security experts break down complex infrastructure topics into practical, beginner-friendly guides.
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