Utilo

CIDR / Subnet Calculator

Calculate network address, broadcast, usable host range, netmask and wildcard for any IPv4 or IPv6 CIDR. Split networks into subnets and check whether an IP is inside a range.

What this CIDR calculator does

Planning a VPC, writing firewall rules or debugging why a container can't reach a database all come down to the same question: which addresses does this CIDR block contain? Enter any IPv4 or IPv6 address with a prefix length, like 10.0.0.0/16 or 2001:db8::/48, and the calculator shows the network address, broadcast address (IPv4), first and last usable host, netmask and wildcard mask, total addresses and usable hosts, address class and scope (private, public, loopback, documentation and more). It can split the network into smaller subnets and check whether a given IP falls inside the range.

How to use it

  1. Type an address with a prefix, e.g. 192.168.1.10/24. A bare IPv4 address defaults to /32.
  2. Read the results table; the binary view shows which bits belong to the network and which to hosts.
  3. Choose a new prefix under split into subnets to list the resulting blocks, such as four /26 subnets inside a /24.
  4. Enter an IP under is an IP in range? to check membership instantly.

Quick reference

  • /32 – a single IPv4 host; /128 for IPv6.
  • /31 – point-to-point link with 2 usable addresses (RFC 3021).
  • /30 – 4 addresses, 2 usable hosts.
  • /24 – 256 addresses, 254 hosts, netmask 255.255.255.0.
  • /16 – 65,536 addresses, a common VPC size.
  • /64 – the standard IPv6 subnet size; SLAAC requires it.
  • /48 – a typical IPv6 site allocation, room for 65,536 /64 subnets.

Private and special ranges

10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 are private (RFC 1918). 100.64.0.0/10 is carrier-grade NAT. 169.254.0.0/16 is link-local. In IPv6, fc00::/7 is unique local, fe80::/10 is link-local and 2001:db8::/32 is reserved for documentation.

Tips

  • AWS reserves 5 addresses in every subnet, Azure 5 and GCP 4, so usable hosts in the cloud are slightly fewer.
  • Avoid overlapping ranges between VPCs, VPNs and Docker's default 172.17.0.0/16 if networks will ever be connected.
  • The wildcard mask is the inverse of the netmask and is used in Cisco ACLs and OSPF.

Frequently asked questions

Why does a /24 have 254 usable hosts, not 256?

The first address identifies the network and the last is the broadcast address, so neither can be assigned to a host. IPv6 has no broadcast.

Does it support IPv6 shorthand?

Yes. Input like 2001:db8::1/64 is expanded, and results show both compressed and expanded forms.

What happens if I enter a host address like 192.168.1.10/24?

The calculator normalizes it to the network 192.168.1.0/24 and still shows where your host sits in the range.

Is anything sent to a server?

No. All calculations use BigInt math in your browser.

Yes. All processing happens directly in your browser using JavaScript, Web Workers and Web APIs. Nothing you type, paste or upload is sent to our servers.