What Is CIDR Notation?
CIDR (Classless Inter-Domain Routing) notation represents an IP address and its associated network prefix as a single compact value — for example 192.168.1.0/24. The number after the slash is the prefix length: the count of consecutive 1-bits in the subnet mask that identify the network portion of the address.
CIDR replaced the older classful system (Class A/B/C) in 1993, enabling more efficient IP address allocation and route aggregation. Today it is the universal standard for configuring subnets in cloud VPCs, on-premises networks, firewalls, and routing protocols.
Understanding Subnet Masks
A subnet mask is a 32-bit number written in dotted-decimal notation. It uses consecutive 1-bits for the network portion and 0-bits for the host portion. The subnet mask 255.255.255.0 is equivalent to /24 — the first 24 bits identify the network and the last 8 bits identify individual hosts within that network.
To find the network address, AND the IP address with the subnet mask. To find the broadcast address, OR the network address with the inverted mask (wildcard mask). Every address between network+1 and broadcast−1 is a usable host address.
Frequently Asked Questions
Note: All calculations use standard IPv4 subnetting rules. AWS subnet usable host counts reflect AWS's reservation of 5 IP addresses per subnet. For production network design, verify results against your platform's documentation.