What Is a Subnet Mask? Explained Simply (2026 Beginner Guide)
Learn what a subnet mask is, how it works with IP addresses, what 255.255.255.0 means, and how subnetting divides networks. Plain English guide with examples.
What Is a Subnet Mask? Explained Simply (2026 Beginner Guide)
You've seen numbers like 255.255.255.0 in your network settings and wondered what they mean. That number is a subnet mask — and it's one of the most misunderstood things in networking. This guide explains it so clearly that you'll actually remember it.
What Is a Subnet Mask?
A subnet mask tells a device which part of an IP address identifies the network and which part identifies the specific device (host) on that network.
Every IP address has two parts: the network part (same for all devices on the network) and the host part (unique to each device). The subnet mask 'masks' the network portion — a bit like highlighting which part of a postcode is the city and which is the street.
Understanding 255.255.255.0
This is the most common home network subnet mask. In binary, 255 = 11111111 — those bits belong to the network. 0 = 00000000 — those bits belong to the host.
Example: IP 192.168.1.105 with mask 255.255.255.0 → network = 192.168.1, host = 105. All devices on your home WiFi share the 192.168.1.x network. Your router is usually 192.168.1.1.
What Is CIDR Notation?
Instead of writing 255.255.255.0, you can write /24. The number after the slash is how many bits are for the network. /24 means 24 bits for the network and 8 bits for hosts — which gives 254 usable host addresses (256 minus the network address and the broadcast address).
- /8 — mask 255.0.0.0 — about 16.7 million usable hosts.
- /16 — mask 255.255.0.0 — 65,534 usable hosts.
- /24 — mask 255.255.255.0 — 254 usable hosts.
- /32 — mask 255.255.255.255 — a single host.
Why Subnetting Exists
Without subnetting, you'd have one giant flat network where every device sees everyone else's traffic — slow, insecure, and impractical at scale.
With subnetting, you divide a large network into smaller logical segments. Each subnet is a separate 'neighbourhood'. In an office, accounts can sit on one subnet and HR on another, so they can't see each other's network traffic.
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Open What Is My IPDefault Subnet Masks by Class
- Class A — 255.0.0.0 (/8) — large networks, millions of hosts (e.g. 10.x.x.x).
- Class B — 255.255.0.0 (/16) — medium networks (e.g. 172.16.x.x).
- Class C — 255.255.255.0 (/24) — small networks, 254 hosts (e.g. 192.168.1.x). Most home networks.
The old class-based system has largely been replaced by classless routing (CIDR), which allows any mask length rather than just /8, /16, /24.
Private vs Public IP Ranges
- 10.0.0.0 – 10.255.255.255 (/8) — large private range.
- 172.16.0.0 – 172.31.255.255 (/12) — medium private range.
- 192.168.0.0 – 192.168.255.255 (/16) — home/SOHO range.
These are never routed on the public internet — your router's NAT translates them to your single public IP when traffic leaves your network.
Practical Subnetting Example
A company with 200 employees uses 192.168.1.0/24 — 254 usable addresses, which is enough. They divide it further into three /26 subnets: Sales on 192.168.1.0/26 (62 hosts), IT on 192.168.1.64/26 (62 hosts), and HR on 192.168.1.128/26 (62 hosts). Now each department has its own isolated segment.
Frequently Asked Questions
What does 255.255.255.0 mean as a subnet mask?
The first three octets of the IP address identify the network and the last octet identifies the individual device. Equivalent to /24 in CIDR notation — 254 usable hosts.
What is the subnet mask for a home network?
Most home networks use 255.255.255.0 (/24), with devices typically on 192.168.1.x or 192.168.0.x.
What is CIDR notation?
CIDR expresses a subnet mask as a slash followed by the number of network bits. /24 is the same as 255.255.255.0, but more compact and flexible than the old classful system.
What is the difference between a subnet mask and an IP address?
An IP address identifies a specific device. A subnet mask tells the device which portion of that IP is the network and which is the host. They always work together.
What is a /32 subnet?
A /32 subnet mask (255.255.255.255) covers a single IP address. It's used in routing rules and firewall configs to reference one specific IP.
Frequently Asked Questions
What does 255.255.255.0 mean as a subnet mask?+
A subnet mask of 255.255.255.0 means the first three octets of the IP address identify the network and the last octet identifies the individual device. This is equivalent to /24 in CIDR notation and allows for 254 usable host addresses.
What is the subnet mask for a home network?+
Most home networks use a subnet mask of 255.255.255.0 (/24), meaning all devices share the same network address (typically 192.168.1.x or 192.168.0.x) and can communicate directly with each other.
What is CIDR notation?+
CIDR (Classless Inter-Domain Routing) notation expresses a subnet mask as a slash followed by the number of network bits — for example, /24 instead of 255.255.255.0. It is more compact and flexible than the older classful system.
What is the difference between a subnet mask and an IP address?+
An IP address identifies a specific device on a network. A subnet mask tells the device which portion of that IP refers to the network and which portion refers to the individual host. They always work together.
What is a /32 subnet?+
A /32 subnet mask (255.255.255.255) covers a single IP address — exactly one host. It is used in routing rules and firewall configurations to reference a specific individual IP with no range.