IPv4 CIDR & Subnet Calculator

What is CIDR and how does subnetting work?

Classless Inter-Domain Routing (CIDR) is the standard method used to allocate IP addresses and route IP packets across the global internet and local computer networks. Before CIDR was introduced in 1993, IPv4 networks were divided into rigid classes: Class A (/8, supporting 16 million hosts), Class B (/16, supporting 65,534 hosts), and Class C (/24, supporting 254 hosts). This inflexible system wasted vast blocks of address space.

CIDR replaced fixed classes with variable-length subnet masking (VLSM). In CIDR notation (such as 192.168.1.0/24 or 10.0.0.0/16), the number following the forward slash denotes the exact number of leading bits assigned to the network prefix. The remaining bits ($32 - \text{prefix}$) are available for assigning to individual host interfaces.

How subnet calculations work

An IPv4 address consists of 32 binary bits, divided into four 8-bit octets separated by dots. When calculating network boundaries:

• Subnet Mask: Created by setting the first $N$ bits to 1 and the remaining bits to 0. For a /24 network, 24 ones followed by 8 zeros produces 255.255.255.0.
• Network Address: Calculated by performing a bitwise logical AND operation between the IP address and the subnet mask. This identifies the base address of the subnet.
• Broadcast Address: Calculated by performing a bitwise logical OR between the IP address and the inverted subnet mask (wildcard mask). Packets sent to this address reach every device on the subnet.
• Usable Host Range: The IP addresses between the network address and broadcast address. For a /24 network (256 total IPs), usable hosts span from .1 to .254 (254 usable hosts).
• Wildcard Mask: The bitwise inverse of the subnet mask, heavily used in Cisco Access Control Lists (ACLs) and firewall rule definitions.

RFC 1918 Private IPv4 address blocks

To preserve public IPv4 addresses, the Internet Engineering Task Force (IETF) and IANA reserved three private address blocks under RFC 1918 that routers never forward onto the public internet:

• 10.0.0.0/8: $10.0.0.0$ to $10.255.255.255$ ($16,777,216$ addresses). Standard for large enterprise networks and Amazon Web Services (AWS) VPCs.
• 172.16.0.0/12: $172.16.0.0$ to $172.31.255.255$ ($1,048,576$ addresses). Commonly used by Docker bridge networks and corporate VPNs.
• 192.168.0.0/16: $192.168.0.0$ to $192.168.255.255$ ($65,536$ addresses). Ubiquitous across home Wi-Fi routers and small office LANs.

Subnetting in cloud architecture (AWS, GCP, Azure)

When creating a Virtual Private Cloud (VPC) in AWS, Azure, or Google Cloud Platform, careful subnet planning avoids address exhaustion. AWS reserves 5 IP addresses in every subnet (the network address, VPC router, DNS resolver, future use, and broadcast address). Using this CIDR calculator ensures you allocate adequate host capacity for auto-scaling server pools, container clusters, and database clusters.