IP Subnet Calculator
Calculate network, broadcast, usable host range and subnet mask from any IPv4 CIDR. Everything is computed in your browser.
Subnet input
Results
- Network address
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- Broadcast address
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- Usable host range
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- Usable hosts
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- Total addresses
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- Subnet mask
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- Wildcard mask
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- CIDR notation
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How the subnet math works
An IPv4 address is a 32-bit number, usually written as four decimal octets. A CIDR prefix (/24) says how many of those 32 bits are fixed as the "network" portion; the remaining bits identify individual hosts inside that network. The network address zeroes out every host bit, and the broadcast address sets every host bit to 1:
192.168.1.10/24 → Mask: 255.255.255.0 (24 network bits, 8 host bits) → Network: 192.168.1.0 → Broadcast: 192.168.1.255 → Usable: 192.168.1.1 – 192.168.1.254 (254 hosts)
The usable host count follows RFC 4632's formula 2^(32 - prefix) - 2, subtracting the network and broadcast addresses from the total block size. Two prefixes are special cases per RFC 3021: a /31 has exactly 2 usable addresses (no broadcast — both addresses in the block are valid hosts, meant for point-to-point links), and a /32 identifies exactly 1 host with no network or broadcast concept at all. This tool builds the address as a real 32-bit integer and runs the mask math with bitwise operators — no string-based octet arithmetic, which is where off-by-one errors on the /31 and /32 edge cases usually creep in.
Frequently asked questions
What does CIDR notation mean?
CIDR (Classless Inter-Domain Routing, RFC 4632) writes a subnet as an IP address followed by a slash and a prefix length, e.g. 192.168.1.0/24. The prefix length is how many leading bits of the 32-bit address are the network portion; the rest are host bits.
Why does a /31 subnet have no broadcast address?
RFC 3021 defines /31 as a special case for point-to-point links: both addresses in the 2-address block are usable as host addresses, since a link with only two devices doesn't need a separate broadcast address. A /32 similarly has no network or broadcast portion at all — it identifies exactly one host.
How is the number of usable hosts calculated?
For a /0 through /30 prefix, usable hosts = 2^(32-prefix) - 2, subtracting the network and broadcast addresses. /31 has exactly 2 usable addresses and /32 has exactly 1, as special cases.
Can I enter a dotted subnet mask instead of a CIDR prefix?
Yes. The prefix field accepts /24, 24, or 255.255.255.0 — all three are parsed and normalized to the same result.