What CIDR notation is
CIDR (Classless Inter-Domain Routing) represents a subnet as an IP address followed by a slash and the number of netmask bits (e.g. 192.168.1.0/24 means the first 24 bits identify the network, and the remaining 8 bits identify hosts).
What it's for
Essential for configuring networks, firewalls and routing rules: knowing how many hosts fit in a subnet, what the broadcast address is, and what the usable IP range is.
Frequently asked questions
Why don't /31 and /32 have "usable" hosts in the traditional sense?
A /32 subnet has only 1 address (used for single-host routes), and a /31 has 2 addresses, both usable point-to-point (RFC 3021) — there are no reserved network/broadcast addresses in these cases.
Does this tool work with IPv6?
No — for now it only calculates IPv4 subnets.