infoOverview

Working out a subnet mask or the usable host range from a /24 or /20 by hand is tedious and easy to get wrong, especially once you're juggling multiple VLANs or VPC subnets at once. This calculator takes any IPv4 or IPv6 address with a CIDR prefix and instantly returns the netmask, network and broadcast addresses, usable host range and count, and the binary breakdown — so you can double-check a subnet design or decode someone else's without reaching for a notepad.

checklistHow to Use

1. 1Pick the protocol, IPv4 or IPv6.
2. 2Type an address into IP Address — you can include a /prefix here too (e.g., 192.168.1.100/24) and it'll populate the CIDR field for you.
3. 3Otherwise, drag the CIDR slider to set the prefix length (e.g., 24) and watch the host range update live.
4. 4Click Generate.

menu_bookSpecifications & Glossary

• CIDR (Classless Inter-Domain Routing): The slash notation pairing an IP address with its prefix length, e.g. 192.168.1.0/24. It replaced the old classful A/B/C addressing scheme, which wasted huge blocks of address space, and lets you carve out a subnet of nearly any size instead of being locked into fixed class boundaries.
• Subnet mask: The bitmask marking where the network portion ends and the host portion begins. A /24 prefix and a 255.255.255.0 mask describe the exact same boundary — CIDR notation and dotted-decimal masks are just two ways of writing the same value.
• Network address: The first address in the subnet, with every host bit set to 0. Routers and configuration files use this to refer to the subnet as a whole, for example in a routing table entry.
• Broadcast address: The last address in the subnet, with every host bit set to 1. IPv6 has no concept of broadcast — it relies on multicast instead, so this field only applies to IPv4.
• Usable host count: For IPv4 this is 2^(host bits) - 2, reserving one address for the network identifier and one for broadcast. IPv6 subnets don't reserve a broadcast address, so the usable count there is simply 2^(host bits).
• IPv4 vs IPv6: IPv4 addresses are 32 bits, written as four dot-separated decimal octets (192.168.1.1). IPv6 addresses are 128 bits, written as eight colon-separated groups of hex digits (2001:db8::1), and the vastly larger address space removes most of the scarcity pressure that drives aggressive subnetting under IPv4.
• Binary representation: The address spelled out as a string of bits, so the boundary between the network portion and the host portion is something you can see, not just calculate.

lightbulbUse Cases

• Planning subnets for a VPC or an on-premises network before you commit to an address plan.
• Double-checking that a firewall or security-group rule's IP range covers (or doesn't accidentally cover) the addresses you expect.
• Sizing CIDR blocks when laying out a VPC on AWS, GCP, or Azure.