What is CIDR Notation?
CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing Internet Protocol packets. It replaced the older classful network design and allows more flexible division of the IP address space. A CIDR address consists of an IP address followed by a slash and a number (called the prefix length) — for example 192.168.1.0/24. The prefix length specifies how many bits of the address identify the network, while the remaining bits identify individual hosts within that network.
Before CIDR (introduced in 1993), the internet used a rigid class system (A, B, C) that wasted enormous address space. CIDR enabled flexible subnetting, dramatically extending IPv4's usable lifespan and forming the mathematical foundation of modern network design, cloud VPC architecture, and firewall rule configuration.
How to Use This CIDR Calculator?
1
Enter your CIDR block
Type any valid IP address with a prefix length (0–32), e.g. 192.168.10.0/24. You can also enter just an IP — the tool defaults to /32.
2
Click Calculate
The tool instantly computes the full network range, broadcast address, subnet mask, wildcard mask, hex representation, and binary breakdown.
3
Read binary notation
The binary display highlights network bits (purple) vs. host bits, giving you a visual understanding of exactly where the boundary falls within the address.
4
Use in your config
Copy the network address, mask, or wildcard directly into router ACLs, firewall rules, cloud VPC settings, or OSPF configurations.
📊 Understanding CIDR Results
Network Address
All host bits set to 0. This identifies the subnet itself and is never assigned to a device.
Broadcast Address
All host bits set to 1. Packets sent here reach every device on the subnet. Not assignable to a host.
Subnet Mask
32-bit number with network bits = 1, host bits = 0. Used in traditional networking and older equipment.
Wildcard Mask
The inverse of the subnet mask (all bits flipped). Required in Cisco ACLs and OSPF configurations.
Hex Representation
Hexadecimal form of the IP and mask. Used in low-level programming, packet analysis, and SNMP MIBs.
Address Scope
Whether the address is Private (RFC 1918), Public, Loopback, or Link-local — determines routability on the internet.
📜 CIDR Quick Reference Table
💡 Real-World Use Cases
🛠️
Cloud VPC Design
AWS, Azure, and GCP require CIDR blocks for VPCs and subnets. /16 for the VPC, /24 per availability zone is a common pattern. Our
Subnet Calculator helps split them further.
🛡️
Firewall ACLs
Cisco and Juniper ACLs use wildcard masks (not subnet masks). This calculator gives you both, letting you copy directly into your access-control rules.
📋
BGP Prefix Aggregation
ISPs aggregate smaller prefixes into supernets (e.g. four /24s → one /22) to reduce routing table size. CIDR math is fundamental to this route summarisation.
🔍
IP Lookup & Threat Intel
Security teams use CIDR notation to block or alert on entire ASN-assigned ranges. Combine results with our
IP Lookup and
Blacklist Checker.
📜 CIDR vs Classful Networking
🛡️ Security Implications of CIDR
CIDR notation is central to network security. Firewall rules expressed as 10.0.0.0/8 block or permit entire private address ranges with a single rule. Zero-trust architectures define micro-segmentation boundaries using /28 or /29 subnets to isolate workloads. Threat intelligence feeds distribute malicious IP ranges in CIDR format, allowing security devices to block thousands of addresses with one entry. Always verify that your CIDR rules don't inadvertently include broadcast or network addresses — use this calculator to confirm the exact range before deploying.
🔗 Related Tools
For subnet splitting (VLSM), use our dedicated Subnet Calculator which adds split-into-N and hosts-to-size modes. For full IP intelligence including ASN and geolocation of a specific address within a CIDR range, use IP Lookup. For IPv6 CIDR, the notation follows the same rules — use our IPv6 Lookup tool for individual address analysis.
What does CIDR stand for? +
CIDR stands for Classless Inter-Domain Routing. It's a method introduced in 1993 (RFC 1519) for allocating IP addresses and routing packets more efficiently than the older classful A/B/C system.
How do I read CIDR notation like 192.168.1.0/24? +
The IP before the slash is the network address. The number after the slash (24) is the prefix length — it means the first 24 bits identify the network, and the remaining 8 bits (2⁸ = 256 addresses) are available for hosts within that subnet.
What is the difference between subnet mask and CIDR? +
They represent the same information in different formats. Subnet mask 255.255.255.0 = CIDR /24. Subnet mask 255.255.0.0 = CIDR /16. CIDR is more compact and flexible; subnet masks are required by some older protocols and equipment.
Why are 2 addresses always reserved in a subnet? +
The first address (network address, all host bits = 0) identifies the subnet. The last address (broadcast, all host bits = 1) sends a message to all hosts simultaneously. Neither can be assigned to a device. Exception: /31 and /32 follow RFC 3021 rules without broadcast reservation.
What is a wildcard mask and why is it different? +
A wildcard mask is the bitwise inverse of a subnet mask (each 0 becomes 1 and vice versa). It specifies which bits of an address to ignore when matching. Wildcard masks are required in Cisco ACLs and OSPF network statements instead of subnet masks.
What is the difference between /24 and /25? +
A /24 has 256 total addresses (254 usable); a /25 has 128 total addresses (126 usable). Increasing the prefix by 1 halves the host space. Two /25 subnets together form one /24 network, useful for splitting a single segment into two separate broadcast domains.
Can I use CIDR for IPv6? +
Yes. IPv6 uses the same slash notation (e.g. 2001:db8::/32). Standard allocations: ISPs receive /32, enterprises /48, individual subnets /64. The enormous 128-bit address space means no need for address conservation — every link gets a /64 by default.
What does /32 mean? +
A /32 identifies a single host (all 32 bits are the network). It's used in host routes, loopback interface configuration, and to permit or deny a single specific IP address in firewall rules. Total addresses = 1, no broadcast, no network address distinction.
What is a /0 route? +
A /0 (0.0.0.0/0) is the default route representing all possible IP addresses — the "catch-all" used in routing tables. Traffic matching no more specific route is forwarded via the /0 route, typically pointing to the ISP's gateway or internet-facing firewall.
What is supernetting? +
Supernetting is the opposite of subnetting — combining multiple smaller networks into one larger CIDR block for routing efficiency. For example, four /24 networks with consecutive addresses can be summarised as a single /22, reducing BGP routing table entries.
How is CIDR used in AWS VPCs? +
AWS requires you to specify a CIDR block (e.g. 10.0.0.0/16) when creating a VPC. Subnets within the VPC each get a smaller CIDR (e.g. /24 per AZ). AWS reserves 5 addresses per subnet (network, broadcast + 3 for AWS internal use), leaving 251 usable hosts in a /24.
What are RFC 1918 private address ranges? +
RFC 1918 defines three private (non-routable) IPv4 ranges: 10.0.0.0/8 (Class A — 16.7M IPs), 172.16.0.0/12 (Class B-style — 1M IPs), and 192.168.0.0/16 (Class C-style — 65K IPs). These are used internally and not routed on the public internet without NAT.
How do I split a /24 into equal subnets? +
To split a /24 into 2 equal subnets, use /25. Into 4: use /26. Into 8: use /27. The pattern is: each additional bit added to the prefix doubles the number of subnets but halves the hosts per subnet. Use our
Subnet Calculator for the full split with all address ranges listed.
What is VLSM? +
Variable Length Subnet Masking (VLSM) allows different subnets within the same network to have different prefix lengths — right-sizing each segment for its actual host requirement rather than forcing uniform division. This is standard modern practice enabled by CIDR.
Is this CIDR Calculator free to use? +
Yes — completely free, no sign-up, no limits, and no data is sent to our servers. All calculations happen instantly in your browser using pure JavaScript mathematics.