IP Subnet Calculator

What this tool is used for

• Working out the network, broadcast and host range for a CIDR block.
• Checking whether an address falls inside a given subnet.
• Converting a prefix length to a dotted mask.
• Planning a split of a block into smaller subnets.
• Deriving the mask, network and broadcast from a prefix in one pass.

Frequently Asked Questions

What input formats are accepted?

CIDR notation such as `192.168.1.0/24`, or an address and dotted-decimal mask separated by whitespace. A mask is converted to a prefix length by counting leading one-bits and rejected if those bits are not contiguous — `255.255.0.255` is not a valid mask.

Why do /31 and /32 report no wasted addresses?

Because the usual network-and-broadcast pair does not apply. A /32 is a single host route, and RFC 3021 allows a /31 to carry two usable addresses on a point-to-point link where a broadcast address has no meaning. Subtracting two would report zero hosts on both.

Is /0 a valid prefix here?

Yes. It is the default route and the calculator handles it specially, because JavaScript takes shift counts modulo 32 — so `0xffffffff << 32` evaluates to `0xffffffff` rather than 0, and a naive contiguity check rejects the one mask that is entirely zero bits.

What does the address-type field tell me?

Whether the address is loopback, RFC 1918 private space, link-local APIPA, multicast, limited broadcast, or globally routable. It is a quick sanity check that the block you are about to hand out is the kind of block you meant.

How do I read the reverse-DNS zone it shows?

It is the `in-addr.arpa` zone whose boundary matches your prefix — one octet for /8 or shorter, two for /16, three below that. Classless delegation for a prefix that is not on an octet boundary needs the RFC 2317 `CNAME` scheme instead.

Why are classes A, B and C still shown?

Classful addressing was superseded by CIDR in 1993, but the class ranges still appear in exam questions, vendor documentation, and older equipment defaults. The value is informational — the prefix length is what actually determines the network.

Common errors and gotchas

• Counting the network and broadcast addresses as usable hosts, which for IPv4 they are not.
• Off-by-one on the prefix, where a /24 and a /25 differ by half the range.
• Assuming a mask can have gaps, when a valid one is a run of ones then zeros.
• Treating a /31 or /32 as invalid, when both have defined uses.
• Applying IPv4 host arithmetic to IPv6, where the conventions differ entirely.