CIDR Calculator
Calculate network address, broadcast address, host range and usable IPs from a CIDR block.
CIDR details
About CIDR Notation
CIDR (Classless Inter-Domain Routing) notation is the standard way to express IP addresses and their network prefixes. It replaces the older classful system (Class A/B/C) with a flexible prefix length that can represent any subnet size. A CIDR address looks like 192.168.0.0/24 where the number after the slash is the prefix length — the number of bits used for the network portion.
Common CIDR ranges
/32— Single host (1 address). Used in host routes, firewall rules for specific IPs./30— 4 addresses, 2 usable. Used for point-to-point links./29— 8 addresses, 6 usable. Small office or DMZ./28— 16 addresses, 14 usable./27— 32 addresses, 30 usable./24— 256 addresses, 254 usable. The classic "Class C" equivalent. Most common LAN size./16— 65,536 addresses. A "Class B" equivalent. Common in larger organisations./8— 16.7 million addresses. A "Class A" equivalent. ISP-level allocations.
Private address ranges (RFC 1918)
Three IPv4 ranges are reserved for private (internal) use and are not routable on the public internet:
10.0.0.0/8— 16.7 million addresses172.16.0.0/12— 1 million addresses192.168.0.0/16— 65,536 addresses
Frequently asked questions
What is CIDR?
CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses. A CIDR block is written as an IP address followed by a / and the prefix length (e.g. 192.168.1.0/24), where the prefix length indicates how many bits are the network portion.