IP Subnet Calculator

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IPv4 / CIDR

IPv4 address (or CIDR like 10.0.0.5/16)

Prefix / subnet mask

🔒 All subnet math runs in your browser — the address you type is never sent anywhere.

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Enter an IP and prefix, then hit Calculate

How to Use the Subnet Calculator

Enter an IPv4 address — e.g. 192.168.1.10 — or paste full CIDR like 10.0.0.5/16 straight into the box.
Pick the prefix from the dropdown (each option shows its dotted mask, e.g. /24 — 255.255.255.0).
Click Calculate to see network, broadcast, usable range, masks, class and the binary breakdown.

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Instant & exact

Pure integer math, no rounding — every value from /0 to /32 including the /31 and /32 special cases.

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Learn while you calc

Binary view of the IP and mask makes CIDR finally click — perfect for CCNA, Network+ and homework.

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Private

Your internal network addresses never leave your browser — nothing is logged or transmitted.

CIDR and Subnetting, Explained

CIDR (Classless Inter-Domain Routing) notation describes a network as an IP address plus a prefix length — 192.168.1.0/24 means the first 24 bits identify the network and the remaining 8 bits identify hosts. The prefix maps directly to a subnet mask: /24 is 255.255.255.0, /16 is 255.255.0.0, /26 is 255.255.255.192. The wildcard mask is simply the inverse (0.0.0.255 for /24) and is what Cisco ACLs and OSPF configuration expect.

Every subnet reserves two addresses: the all-zeros network address and the all-ones broadcast address. That is why a /24 offers 2^8 − 2 = 254 usable hosts and a /30 only 2. Two modern exceptions matter in 2026 networks: a /31 (RFC 3021) provides 2 usable addresses for point-to-point router links with no broadcast, and a /32 identifies a single host — the standard form for firewall rules, loopbacks and route advertisements.

The calculator also identifies the historical class (A: 1–126, B: 128–191, C: 192–223, D multicast, E experimental) and whether the address is private or public. The RFC 1918 private ranges — 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 — are what your home router, office LAN and cloud VPCs use internally; they are never routed on the public internet. It also flags loopback (127/8), link-local/APIPA (169.254/16) and carrier-grade NAT (100.64/10) space, so you can tell at a glance why a device got a weird address.

Whether you are carving an AWS or Azure VPC into subnets, sizing a VLAN, studying for the CCNA, or just figuring out which addresses are free on your home network, all of the math here is exact 32-bit integer arithmetic done locally in your browser.

IP Subnet Calculator FAQ

What is CIDR notation? ▾

CIDR (Classless Inter-Domain Routing) writes a network as an IP plus a prefix length, like 192.168.1.0/24. The prefix says how many leading bits identify the network; the remaining bits identify hosts within it.

How many usable hosts are in a /24? ▾

254. A /24 has 256 addresses, but the first (network address) and last (broadcast address) are reserved, leaving 2^8 minus 2 usable host addresses.

What is the difference between a subnet mask and a wildcard mask? ▾

They are bitwise inverses. A /24 subnet mask is 255.255.255.0 while its wildcard mask is 0.0.0.255. Wildcard masks are used in Cisco access lists and OSPF network statements.

Which IP addresses are private? ▾

The RFC 1918 ranges: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. They are used inside homes, offices and cloud VPCs and are never routed on the public internet.

Why does a /31 show 2 usable hosts and no broadcast? ▾

RFC 3021 allows /31 subnets on point-to-point router links, where both addresses are usable and no broadcast address is needed. A /32 identifies a single host, commonly used in firewall rules and loopbacks.

Is the IP address I enter sent anywhere? ▾

No. All subnet math is pure 32-bit integer arithmetic done locally in your browser, so internal network addresses stay completely private.

What are IP address classes? ▾

The historical classful system: Class A (1–126), B (128–191) and C (192–223) for unicast, D (224–239) for multicast and E (240–255) reserved. Modern networks use CIDR instead, but classes still appear on certification exams.

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