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Tools

IP Calculator

IP address and anonymity

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Subnets, masks and CIDR ranges.

What a subnet calculator works out

From an address and a mask every network parameter follows. You can do it in your head, but one slip costs an hour of debugging.

Network address The first address in the range. It is never assigned to a device; it identifies the network itself.
Broadcast address The last address. A packet sent there reaches every host at once, so it is reserved as well.
Usable host range Everything between the first and last. These are the addresses handed out to servers, computers and printers.
Number of hosts How many devices fit. The formula is simple: two to the power of the free bits, minus the two reserved addresses.
The mask in each notation 255.255.255.0 and /24 say the same thing differently. Equipment accepts one form or the other.

The masks you meet most

A guide to how many devices each one holds.

/30 Two usable addresses. The classic point-to-point link between two routers.
/29 Six addresses. The typical block a host allocates for a handful of servers.
/24 254 addresses. The office-network standard and the one you will handle most often.
/16 65,534 addresses. A large corporate network, usually split into smaller ones.

Where people get it wrong

Forgetting the two reserved addresses A /24 has 254 slots, not 256. The first and last are always reserved, and planning trips over this constantly.
Overlapping ranges Two subnets whose ranges overlap create conflicts that surface late and take a long time to unpick.
Taking a range “just in case” A /16 allocated for twenty devices. You will not run out, but routing and security get needlessly complicated.
Confusing mask with range The mask says how many bits belong to the network, not which address comes first. Firewall rules go wrong here.

IP subnet calculator (CIDR)

Enter an IP address and mask (CIDR prefix from /0 to /32) — the calculator instantly computes the network address, broadcast, dotted-decimal mask, wildcard, usable host range and host count. All calculations run in your browser, with no server requests.

The tool is indispensable for sysadmins and anyone configuring networks: split a range into subnets, count addresses for an office, correctly write firewall rules or routing. No more powers of two in your head — the subnet calculator does it without mistakes.

FAQ

What are CIDR and subnet mask?
CIDR (e.g. /24) is a short mask form defining how many address bits are for the network. /24 = 255.255.255.0 and gives 254 hosts.
How to find the number of hosts?
Formula: 2^(32 − mask) − 2. For /24 that is 2^8 − 2 = 254 addresses (two subtracted for network and broadcast).
What is a broadcast address?
It is the last address of a subnet to which messages are sent to all network devices at once. It is not used for hosts.
Why use an IP calculator?
To plan networks, configure routers and firewalls, split a range into subnets without calculation errors.
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