Read a CIDR the way a network change needs it
A security-group rule, a VPC block, or an nginx allowlist is usually a CIDR. This calculator turns 10.0.0.0/24 into network, broadcast, first and last host, usable count, mask, and wildcard.
The second box checks overlap. The sample 10.0.0.0/24 and 10.0.0.128/25 overlap because the /25 sits inside the /24.
Addressing rules are in RFC 4632.
Network, broadcast, and usable hosts
For prefixes 0-30 the network and broadcast addresses are reserved, so usable hosts are 2^(32-prefix) - 2. /24 is 256 addresses and 254 hosts.
Overlap checks for VPC and allowlists
Two ranges overlap when each start is less than or equal to the other end. Use this before you attach a second CIDR to a VPC or a VPN.
IPv4 notes (/31 and /32)
/32 is one host. /31 is two usable addresses for a point-to-point link (RFC 3021). The calculator does not subtract network and broadcast on those prefixes.
IPv6 input is accepted and checked, but it does not compute a host list.
How to use
- Enter a CIDR or an IP plus dotted mask.
- Optionally enter a second CIDR for overlap.
- Copy the summary.
Privacy
All math is local.
Frequently Asked Questions
What does /31 mean?
RFC 3021 uses /31 for point-to-point links. Both addresses are usable. There is no separate broadcast or discarded network address.
What about /32?
A /32 is a single host. Network, first host, and last host are the same address. Usable host count is 1.
Does IPv6 show a host range?
No. IPv6 is a parse check only. Host-range and overlap math are implemented for IPv4.
Is this a port scanner or whois lookup?
No. It is integer math on the addresses you type.
Is anything sent to a server?
No.