Subnet calculator

They describe the same thing. CIDR writes how many leading bits belong to the network (/24); a subnet mask writes those bits out as an address (255.255.255.0). The calculator accepts either: 10.0.0.5/24 and 10.0.0.5 255.255.255.0 give the same result.

The inverse of the subnet mask: 0.0.0.255 for a /24. Cisco access lists and OSPF configuration use wildcard masks, where a 0 bit means "must match" and a 1 bit means "any value".

A /31 has only two addresses, so reserving a network and a broadcast address would leave none. RFC 3021 allows both to be used on point-to-point links between two routers. A /32 is a single host, used for loopbacks and host routes.

Choose "IP range to CIDR", enter the first and last address, and the calculator returns the fewest CIDR blocks that cover exactly that range. A range that starts and ends on block boundaries, such as 192.168.0.0 to 192.168.0.255, collapses into one block (192.168.0.0/24); others need several.

IPv6 addresses are 128 bits, so the same prefix maths applies on a much larger scale. A /64 is the standard size for one network segment because address autoconfiguration (SLAAC) uses the last 64 bits for the interface. Providers usually hand customers a /48 or /56, which the calculator shows as 65,536 or 256 networks of /64.

You typed an address inside the network rather than the network address itself, for example 192.168.1.77/24. The calculator shows the network it belongs to (192.168.1.0/24). Most routers and firewalls expect the network address when you configure a route or rule.

Websites often rate-limit or block traffic by /24, because addresses in one /24 usually belong to the same network and owner. If many proxies sit in one shared /24, one abusive neighbour can get the whole block flagged. Owning an entire /24 means no neighbours on the block can affect its reputation.

No. Every calculation runs in your browser, so you can use the calculator for private and internal address plans.