CIDR Calculator
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Enter a CIDR block and get everything that follows from it: network and broadcast addresses, netmask and wildcard, the usable host range, and the address count. It handles the two cases the textbook formula fails on — a /31, which RFC 3021 gives two usable addresses and no broadcast, and a /32, which is a single host — instead of reporting zero or minus one hosts.
How to use CIDR Calculator
- 1
Enter a block in CIDR notation, e.g. 10.0.0.0/16, or a host address with its prefix.
- 2
Read the network, broadcast, mask and usable range.
- 3
Copy the result — the tool says plainly when the address you entered is a host rather than the network.
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Frequently asked questions
What does the number after the slash mean?+
How many bits from the left are the network part. A /24 fixes the first 24 bits, leaving 8 for hosts — 256 addresses, 254 of them usable. Every bit you add to the prefix halves the block, so /25 is 128 addresses and /23 is 512.
Why does a /31 show 2 usable hosts?+
Because RFC 3021 says so. The old formula subtracts a network and a broadcast address from every block, which leaves a /31 with zero — yet /31s are used routinely on point-to-point links, where both addresses are hosts and there is no broadcast. A calculator that reports 0 is quoting arithmetic rather than describing a network.
I entered 192.168.1.50/24 and got 192.168.1.0.+
That is correct, and it is worth saying out loud: the network address is the input with the host bits zeroed. 192.168.1.50 is a host inside 192.168.1.0/24. The tool reports both so nobody leaves believing their host address was the network.
What is the wildcard mask for?+
It is the bitwise inverse of the netmask, and it is what Cisco ACLs and OSPF configuration expect instead of a netmask. 255.255.255.0 becomes 0.0.0.255. Same information, opposite convention, and mixing them up is a long-standing source of ACLs that match nothing.
Does this handle IPv6?+
No — this is IPv4. IPv6 subnetting is a different exercise in practice: the convention is a /64 per LAN regardless of how many hosts are on it, so the host-count arithmetic that makes an IPv4 calculator useful mostly does not arise.