Convert CIDR notation into its first and last IP address, usable host range, and subnet masks.
/31 point-to-point link (RFC 3021): both addresses are usable, no broadcast.
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CIDR notation compresses a whole address block into one expression:
address/prefix-length. The prefix says how many of the 32 bits in an IPv4 address
identify the network; the remaining bits enumerate hosts. Converting to a range is pure bit
arithmetic — clear the host bits for the first address, set them for the last — and that's
exactly what this page does, server-side, using the same math engine as our main subnet
calculator.
The conversion matters most where tooling expects ranges instead of prefixes: firewall vendors
that want start–end pairs, IP allowlists in SaaS admin panels, database BETWEEN
queries on integer-encoded addresses, and capacity questions ("does 10.4.0.0/14 cover our
260,000 devices?" — it has 262,144 addresses, so barely). The wildcard mask shown is what
Cisco ACLs and OSPF network statements expect — the bitwise inverse of the subnet mask.
Watch for the alignment rule: a CIDR block always starts on a multiple of its own size. If the range you got starts before the address you typed, your input wasn't the network address of the block — the tool aligned it for you and the note shows the true boundaries. When you need the opposite conversion (an arbitrary start–end range expressed as CIDR blocks), that generally requires several prefixes; our supernet calculator handles that aggregation.
Split the notation at the slash: the prefix length says how many leading bits are fixed. For 10.0.0.0/24, the first 24 bits are the network, leaving 8 host bits — so the range runs from 10.0.0.0 (all host bits 0) to 10.0.0.255 (all host bits 1), 256 addresses in total. This tool does the bit math for any prefix from /1 to /32.
In a normal subnet the first address (network address) and last address (broadcast) can't be assigned to hosts, so a /24 has 256 addresses but 254 usable hosts. Exceptions: /31 point-to-point links use both addresses (RFC 3021), and /32 is a single host route.
CIDR blocks are aligned: the network address is the entered IP with all host bits cleared. If you enter 10.0.0.55/24, the block is still 10.0.0.0 – 10.0.0.255, because /24 boundaries fall on multiples of 256. The tool shows the range of the block containing your address.
Total addresses = 2^(32 − prefix). A /16 has 65,536 addresses (65,534 usable), a /24 has 256 (254 usable), and a /28 has 16 (14 usable). Each step down in prefix length doubles the size.
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