Subnet / CIDR Calculator
IPv4 and IPv6 subnet maths, all in your browser
🧮 Subnet / CIDR Calculator
Enter an IPv4 or IPv6 CIDR range. We compute the network address, broadcast, host range, total addresses, and subnet mask. Everything happens in your browser. No data leaves the page.
Try an example: 10.0.0.0/24 · 192.168.1.0/26 · 2001:db8::/48
Frequently Asked Questions
Common questions about CIDR notation and subnetting.
What does CIDR notation mean?
CIDR (Classless Inter-Domain Routing) writes a subnet as address/prefix-length. The prefix length is the number of leading bits that identify the network. The remaining bits identify hosts. 192.168.1.0/24 means "the first 24 bits are the network, the last 8 bits are the host," which gives 256 addresses (254 of them usable).
How do I write CIDR in an SPF record?
Use ip4: or ip6: followed by the CIDR. ip4:192.0.2.0/24 authorises that whole /24. ip6:2001:db8::/32 authorises a /32 of IPv6. Fewer, larger ranges keep your SPF short and away from the 10-DNS-lookup limit.
What's the difference between /30 and /31?
A /30 has 4 addresses: network, two usable hosts, and broadcast. A /31 (RFC 3021) has just 2 addresses, both usable for point-to-point links, with no separate network or broadcast address. Most modern routers handle /31. For host networks, stick with /30 unless you specifically need a point-to-point link.
Why does IPv6 not have a broadcast address?
IPv6 dropped broadcast in favour of multicast. The "all hosts on this link" function is handled by the link-local multicast group ff02::1. So an IPv6 subnet has a network address and a usable range, but no single broadcast address. Every address in the prefix is a usable host address.
Is this calculation private?
Yes. The calculator runs in your browser using JavaScript and BigInt. Nothing about your CIDR input is sent to our server, logged, or stored. You can confirm this by opening your browser's Network tab while you click Calculate.