Subnetting questions always reduce to the same six answers: where does this network start, where does it end, what mask defines it, and how many hosts fit inside? This IP subnet calculator gives you all of them from a single CIDR entry — network address, broadcast address, subnet mask, wildcard mask, first and last usable hosts, and the total and usable address counts.
Type an address like 192.168.1.0/24 (a dotted mask like 192.168.1.10 255.255.255.0 works too) and the table fills in as you type. All arithmetic is done locally in your browser — useful when the addresses you are planning are internal and not meant to leave the building.
CIDR notation, decoded once and for all
The number after the slash is the prefix length: how many of the address’s 32 bits belong to the network. /24 means the first 24 bits name the network and the last 8 are free for hosts — 256 addresses. Every step down doubles the network: /25 has 128 addresses, /26 has 64, and so on down to /32, a single host route. The subnet mask is the same information drawn as an address: /24 becomes 255.255.255.0, and the wildcard mask is its bitwise inverse (0.0.0.255), which is what Cisco ACLs ask for.
A worked example
The calculator opens on 192.168.1.0/24: network 192.168.1.0, broadcast 192.168.1.255, and usable hosts .1 through .254 — 254 of them, because the first address names the network itself and the last is the broadcast address where packets address everyone at once. Now change the entry to 172.16.5.4/30 and watch the calculator normalise it: the network is 172.16.5.4, broadcast .7, and exactly two usable hosts (.5 and .6) — which is why /30 is the classic size for a point-to-point link between two routers.
The special cases worth knowing
/31 breaks the “minus two” rule on purpose: RFC 3021 allows both addresses on a point-to-point link to be hosts, and the calculator reports 2 usable. /32 is a single address — a host route, used for loopbacks and anycast. Host bits set: if you type 192.168.1.77/24, the calculator quietly computes the network the address belongs to (192.168.1.0/24) rather than scolding you, because in practice you usually want the containing network. And the range-type row flags RFC 1918 private space (10/8, 172.16/12, 192.168/16) plus loopback, so a plan that accidentally assumes private addresses are routable on the internet gets caught early.
How to use it when planning
Work from the host count backwards. Need room for 50 devices with growth? 26 = 64 addresses means /26, minus network and broadcast leaves 62 usable. Need several subnets inside a /24? Each split halves the space: two /25s, four /26s, eight /27s. Type the parent network into the calculator, read its range, and carve from the boundaries it shows — subnets must start on their own size’s boundary, which the network row confirms for any candidate address you test.
Frequently asked questions
Why is the usable count always total minus 2?
Because two addresses in every classic subnet are spoken for: the network address (all host bits zero) names the subnet itself, and the broadcast address (all host bits one) targets every host. The exceptions are /31 point-to-point links and /32 host routes, which the calculator handles explicitly.
Can I enter a mask instead of a prefix?
Yes: 192.168.1.10 255.255.255.0. The calculator converts the mask to a prefix and validates that it is contiguous — a mask like 255.255.0.255 is not a real subnet mask and is rejected.
Does it support IPv6?
No. IPv6 subnetting follows different conventions (there is no broadcast, and /64 is the near-universal LAN size), so it deserves its own tool rather than a half-fitting mode in this one.
Are the addresses I type uploaded?
No. The calculation is integer arithmetic in your browser tab.
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Versión en español: Calculadora de Subredes IP CIDR Máscara de Red Gratis: Red, Broadcast y Hosts sin Instalar Nada. You may also like: DNS Record Checker — Look Up A, AAAA, MX, TXT, CNAME and NS Records · User-Agent Parser — Read Any User Agent String Online.
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