CIDR Calculator
Instant subnet calculations with interactive binary visualization.
Binary Visualization
Network Range
Summary
| Network | First | Hosts | CIDR |
|---|
What is CIDR?
Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and routing that replaced the classful network architecture. It uses a prefix-length notation (e.g., 192.168.1.0/24) to specify the network portion.
CIDR vs Subnet Mask
CIDR notation (/24) and subnet mask (255.255.255.0) are two ways of expressing the same information. The prefix length is the number of consecutive 1-bits in the subnet mask.
IPv4 Classes
Class A (1-126, /8), Class B (128-191, /16), and Class C (192-223, /24) define default network boundaries. CIDR replaced this rigid system with variable prefix lengths.
Reserved IP Ranges
Private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are not routed on the internet. Other reserved ranges include loopback (127.0.0.0/8) and link-local (169.254.0.0/16).
Binary Explained
Every IPv4 address is a 32-bit binary number. The first N bits (the prefix) identify the network, the remaining 32-N bits identify the host on that network. Our binary visualization makes this clear.
Common Prefixes
Frequently Asked Questions
How does CIDR notation work?
CIDR notation represents an IP address followed by a slash and the prefix length (e.g., 192.168.1.0/24). The prefix length indicates how many of the 32 bits are used for the network portion. The remaining bits identify hosts on that network. This flexible system replaced the rigid class-based addressing (A, B, C) and allows efficient allocation of address space.
How many usable hosts in common CIDR blocks?
/24 = 254 hosts, /25 = 126 hosts, /26 = 62 hosts, /27 = 30 hosts, /28 = 14 hosts, /29 = 6 hosts, /30 = 2 hosts (point-to-point), /32 = 1 host (single device). The formula is 2(32-prefix) - 2 (subtracting the network and broadcast addresses). For /31, there are 2 usable hosts (no network/broadcast reserved).
What is the difference between public and private IPs?
Public IP addresses are globally unique and routable on the internet. Private IP addresses (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are only valid within local networks and are not routed on the internet. NAT (Network Address Translation) allows private IPs to access the internet through a public IP.
What is a subnet mask?
A subnet mask is a 32-bit number that divides an IP address into network and host portions. Network bits are set to 1, host bits to 0. For example, 255.255.255.0 (binary: 24 ones followed by 8 zeros) means the first 24 bits identify the network. The subnet mask is equivalent to CIDR prefix /24.
Network Tools
Advanced tools for network engineers
Host Requirement Calculator
Find the minimum CIDR prefix for a given number of hosts.
CIDR Comparison
Check overlap and containment between two subnets.
Supernet Calculator
Find the smallest summary route for multiple CIDRs.
Subnet Splitter
Split a network into equal-sized subnets.
VLSM Calculator
Variable Length Subnet Masking with host requirements.
Subnet Mask Lookup
Complete reference table from /1 to /32.
| CIDR | Mask | Hosts |
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IPv6 & Cloud Tools
IPv6 calculations and cloud subnet planning
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Calculate network details for any IPv6 address.
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Convert between IPv4 and IPv6 formats.
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Plan VPC subnets across Availability Zones.
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Plan Azure Virtual Network subnets.
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Plan Google Cloud VPC subnets by region.
Terraform Generator
Generate Terraform configs from CIDR blocks.
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Design network segments and generate firewall rules
Network Segments
Define segments with host requirements.
ACL Generator
Generate ACL rules for any platform.
Bulk Calculator
Calculate hundreds of CIDRs at once.
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