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What Is Subnetting?

Subnetting is the process of taking one large IP network and splitting it into smaller, self-contained pieces called subnets. Every IPv4 address is really two parts glued together — a network portion and a host portion — and the subnet mask (or CIDR prefix, like /26) is what tells a router where that split happens.

Think of a large office building with a single street address. Subnetting is like assigning each floor its own suite number — mail still reaches the same building, but now it can be routed to the right floor without every piece of mail needing to be sorted by hand.

Why Subnetting Is Used

• Efficient address allocation. Instead of wasting a full /24 (254 usable hosts) on a point-to-point link that only needs 2 addresses, you can carve out a /30 and use the rest elsewhere.
• Smaller broadcast domains. Broadcast traffic stays inside its own subnet, so a large flat network doesn't slow down every device with traffic it doesn't need.
• Security and segmentation. Separating departments, VLANs, or trust zones onto different subnets makes it much easier to apply firewall rules and ACLs between them.
• Cleaner routing. Well-planned subnets can often be summarized into a single route advertisement, keeping routing tables smaller and easier to troubleshoot.
• Fault isolation. A Layer 2 issue on one subnet is contained instead of taking down the entire network.

Key Terms to Know

• Network address — the first address in a subnet, used to identify the subnet itself. Never assigned to a host.
• Broadcast address — the last address in a subnet, used to reach every host on it at once. Also never assigned to a host.
• Usable host range — every address between the network and broadcast address; the ones you can actually assign to devices.
• CIDR notation — the /n shorthand for a subnet mask, where n is the number of network bits.
• Wildcard mask — the inverse of the subnet mask, used in Cisco ACLs and OSPF network statements.
• VLSM — using different subnet sizes within the same network to match the actual number of hosts each segment needs.

Quick CIDR Reference