CIDR IP Range Calculator
Calculate IPv4 subnet ranges, network addresses, broadcast boundaries, usable hosts, and binary bitmasks.
Understanding Classless Inter-Domain Routing (CIDR)
CIDR was introduced in 1993 by the IETF to slow the rapid depletion of IPv4 addresses and control the exponential growth of global Internet routing tables. By allowing variable-length subnet masks (VLSM), organizations can allocate exactly the IP space they need for cloud VPCs, Kubernetes pods, and office networks.
Network Engineering Scenarios
- ✓AWS, GCP & Azure VPC Cloud Network Planning
Architect non-overlapping Virtual Private Cloud subnets (such as 10.0.0.0/16 divided into /24 availability zones) for high availability clusters and container services.
- ✓Kubernetes Pod & Service CIDR Allocation
Verify node IP capacities and prevent RFC 1918 address exhaustion when configuring CNI plugins like Calico, Flannel, or Cilium.
- ✓Enterprise Firewall & Access Control Lists (ACLs)
Derive accurate wildcard masks and CIDR prefixes required for Cisco, Juniper, pfSense, and AWS Security Group routing rules.
- ✓CCNA & Network Engineering Exam Prep
Master Classless Inter-Domain Routing mathematics, binary bit shifting, and host calculation formulas for networking certification exams.
Key Capabilities
Full IPv4 Address Hierarchy
Instantly computes Network IP, Broadcast IP, First Usable Host, and Last Usable Host for any prefix from /0 to /32.
Visual Bitmask & Binary Representation
See full 32-bit binary octets color-coded to distinguish network routing bits from assignable host bits.
Subnet Partitioning Explorer
Automatically previews the division of your current CIDR block into child subnets with one click.
Scope & RFC 1918 Identification
Classifies addresses across Public Internet, RFC 1918 Private, Loopback, Link-Local APIPA, and legacy class boundaries.
Frequently Asked Questions
What does CIDR stand for and how does it work?
CIDR stands for Classless Inter-Domain Routing. It replaces the old rigid Class A, B, and C system with flexible prefix routing. The number after the slash (like `/24`) indicates how many of the 32 bits belong to the network prefix, leaving the remaining bits for individual host devices.
Why are 2 addresses deducted from usable hosts?
In standard IPv4 subnets, the very first address represents the Network Identifier (wire address) and the very last address is reserved as the Broadcast Address (delivering packets to all hosts on the subnet). Hence, usable hosts equal 2^(32 - prefix) - 2.
How many IP addresses are in a /24 and /16 subnet?
A `/24` subnet contains 256 total addresses with 254 usable hosts (e.g. 255.255.255.0). A `/16` subnet contains 65,536 total addresses with 65,534 usable hosts (e.g. 255.255.0.0).
What is an RFC 3021 /31 subnet used for?
RFC 3021 allows point-to-point links (such as router-to-router connections) to use `/31` subnets with exactly 2 usable addresses, eliminating the need for separate network and broadcast addresses to conserve IPv4 space.
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