Subnet Calculator
Calculate network address, broadcast address, subnet mask, and usable host range instantly from any IP address and CIDR prefix.
Enter an IPv4 address with a CIDR prefix above (e.g. 10.0.0.0/16) to calculate its subnet details.
What Is CIDR Notation?
CIDR (Classless Inter-Domain Routing) notation is a method of representing an IP address and its routing prefix length by appending a slash and a number to the address, such as 192.168.1.0/24. That number — the prefix length — is how many leading bits of the 32-bit IP address are fixed as the "network" portion; the remaining bits identify individual hosts within that network.
CIDR replaced the older "classful" system (Class A/B/C), which wasted huge blocks of address space by only allowing fixed-size networks. CIDR lets a network be sized to fit exactly what's needed, from a single host (/32) up to millions of addresses (/8).
Why Subnetting Matters
- Network segmentation: Splitting a network into subnets isolates traffic between departments, VLANs, or environments (production vs. staging), limiting how far a security incident can spread.
- Cloud infrastructure & certification: AWS VPCs, Azure Virtual Networks, and Google Cloud VPCs are all defined by a CIDR block, and every subnet within them needs its range planned to avoid overlap. Subnetting is also a foundational topic on industry exams like Cisco CCNA, AWS Certified Solutions Architect, and CompTIA Network+.
- Firewall rules: Precise CIDR ranges let you scope a firewall or security group rule to exactly the hosts that need access, instead of opening it to a whole network.
- Efficient address use: Sizing a subnet to the number of hosts it actually needs — instead of defaulting to /24 everywhere — avoids wasting IP address space, which matters most with limited private ranges or public IPv4 blocks.
Understanding Your Results
| Field | What It Means |
|---|---|
| Network Address | The first address in the subnet (all host bits set to 0). Identifies the subnet itself — not assignable to a device. |
| Broadcast Address | The last address in the subnet (all host bits set to 1). Used to reach every device on the subnet at once — not assignable to a device. |
| Usable Host Range | The addresses between the network and broadcast address — these are the ones you can actually assign to devices. |
| Subnet Mask | The dotted-decimal form of the CIDR prefix (e.g. /24 = 255.255.255.0), used to configure network interfaces. |
| Wildcard Mask | The inverse of the subnet mask, used in Cisco access control lists (ACLs) and some routing configurations. |
| IP Class | The legacy Class A/B/C/D/E designation based on the first octet. Informational only — CIDR ignores class boundaries. |
| Address Type | Whether the address falls in a private (RFC 1918), loopback, link-local, or public range. |
Common CIDR Prefixes Reference
Quick reference for the most commonly used subnet sizes:
| CIDR Prefix | Subnet Mask | Total IP Addresses | Usable Hosts |
|---|---|---|---|
/8 | 255.0.0.0 | 16,777,216 | 16,777,214 |
/16 | 255.255.0.0 | 65,536 | 65,534 |
/24 | 255.255.255.0 | 256 | 254 |
/25 | 255.255.255.128 | 128 | 126 |
/26 | 255.255.255.192 | 64 | 62 |
/27 | 255.255.255.224 | 32 | 30 |
/28 | 255.255.255.240 | 16 | 14 |
/29 | 255.255.255.248 | 8 | 6 |
/30 | 255.255.255.252 | 4 | 2 |
/31 | 255.255.255.254 | 2 | 2 (point-to-point, RFC 3021) |
/32 | 255.255.255.255 | 1 | 1 (single host) |
All calculations on this page run entirely in your browser. Need to find the owner of a specific IP address instead? Try the free WHOIS Lookup tool to identify domain ownership and network providers.