Subnet calculator (IPv4, CIDR, wildcard)
A subnet calculator takes an IPv4 address and a prefix length (or dotted mask) and returns the network address (address AND mask), broadcast address (network OR inverted mask), the usable host range (network+1 to broadcast−1), the number of hosts (2^(32−prefix) − 2, except /31 = 2 and /32 = 1), and the wildcard mask (inverted mask, used in Cisco ACLs and OSPF network statements).
| Network | 192.168.1.128/26 |
|---|---|
| Broadcast | 192.168.1.191 |
| Usable hosts | 192.168.1.129 – 192.168.1.190 (62) |
| Subnet mask | 255.255.255.192 |
| Wildcard mask | 0.0.0.63 |
| Total addresses | 64 |
| Class / type | C · Private (RFC 1918) |
| Mask (binary) | 11111111.11111111.11111111.11000000 |
| Network (binary) | 11000000.10101000.00000001.10000000 |
How this works
The calculator does the same binary arithmetic you must do by hand on the exam: it converts the address and mask to 32-bit integers, ANDs them for the network, ORs the network with the inverted mask for the broadcast, and counts hosts as 2^(32−prefix) minus network and broadcast. /31 point-to-point links follow RFC 3021 (2 usable), /32 is a single host. Class and private-range checks follow RFC 1918. Nothing is fetched from a server; the code runs in your browser and is covered by unit tests for /26, /31, /32 and /0.
Worked examples
192.168.1.130/26 → mask 255.255.255.192, block size 64 in the 4th octet, network 192.168.1.128, broadcast 192.168.1.191, hosts .129–.190 (62), wildcard 0.0.0.63.
10.20.37.9/21 → mask 255.255.248.0, block size 8 in the 3rd octet, network 10.20.32.0, broadcast 10.20.39.255, 2,046 hosts, wildcard 0.0.7.255.
Exam shortcut: find the interesting octet (the one where the mask is not 0 or 255), compute block size = 256 − mask octet, and count up in blocks until you pass the address; the previous multiple is the network.
Study the topic
- Network Fundamentals (20% of the exam) · practice questions
- IP Connectivity (25% of the exam) · practice questions
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Primary references
- RFC 4632 — Classless Inter-domain Routing (CIDR) — IETF · government · retrieved 2026-09-21, verified 2026-09-21
- RFC 3021 — Using 31-Bit Prefixes on IPv4 Point-to-Point Links — IETF · government · retrieved 2026-09-21, verified 2026-09-21
- RFC 1918 — Address Allocation for Private Internets — IETF · government · retrieved 2026-09-21, verified 2026-09-21
- CCNA Exam v1.1 (200-301) Exam Topics — Cisco Systems · primary · retrieved 2026-09-21, verified 2026-09-21