What Is a Subnet?
a Subnet — a smaller network created by dividing a larger IP network — grouping devices that share the same network portion of their address and communicate directly at Layer 2.
How it works
Subnetting splits one big network into several smaller ones by extending the network portion of the address (borrowing host bits). Devices in the same subnet reach each other directly; reaching another subnet requires routing through a gateway. The subnet's size is defined by its mask (a /24 holds 254 hosts, a /26 holds 62).
Why it matters
Subnets are how every network is organised — by department, floor, function or site — for security, performance and manageability. Mastering how to size and calculate them (subnetting) is the single most important CCNA skill. Practise with the subnet calculator.
Frequently asked questions
What is a subnet?
A subdivision of a larger IP network — a group of devices sharing the same network portion of their addresses that can communicate directly without routing.
Why divide a network into subnets?
For security (isolating groups), performance (smaller broadcast domains) and organisation (grouping devices logically) — plus efficient use of address space.
How do devices in different subnets communicate?
Through a router or Layer 3 device — traffic between subnets must be routed, unlike same-subnet traffic which is delivered directly.
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