Free CIDR & Subnet Calculator
Calculate subnet details from any CIDR notation — IPv4 and IPv6 supported. Get network address, broadcast, host range, usable hosts, and AWS/GCP reserved counts instantly in your browser.
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Last updated: May 22 2026
Reviewed by the QuickTooly Team
Subnet Calculator Guide
Why Use QuickTooly's CIDR Subnet Calculator?
- IPv4 & IPv6 support: Works with both address families — enter any CIDR range and get full details instantly.
- Cloud-aware host counts: Shows AWS and GCP usable host counts, accounting for their reserved IP addresses.
- Runs in your browser: No server calls — all calculations happen locally, keeping your network info private.
- Copy any value: Click any result to copy it directly to your clipboard.
- Edge cases handled: Correctly handles /31 (point-to-point), /32 (host routes), /0 (default route), and full IPv6 ranges.
- 100% free: No account, no watermark, no rate limits.
What is CIDR Notation?
CIDR (Classless Inter-Domain Routing) notation represents an IP address and its associated network prefix. It takes the form IP/prefix, where the prefix length indicates how many bits are used for the network portion. For example, 192.168.1.0/24 describes a network with 256 total addresses (2⁸ host bits), of which 254 are usable by hosts.
CIDR replaced the older classful addressing system (Class A/B/C) in 1993, enabling more efficient allocation of the IPv4 address space. Today it is universally used in routing tables, firewall rules, VPC configurations, and network documentation.
AWS & GCP Reserved IP Addresses
Cloud providers reserve several IP addresses within each subnet, reducing the number available to your workloads:
- AWS reserves 5 addresses per subnet: network address, VPC router, DNS server, future use, and broadcast address.
- GCP reserves 4 addresses per subnet: network address, default gateway, second-to-last address (Google's use), and broadcast address.
This matters when sizing subnets — a /28 gives you 16 total IPs but only 11 usable hosts on AWS (16 − 5) and 12 on GCP (16 − 4).
How to Use This Subnet Calculator
- Type your CIDR range into the input field (e.g.
10.0.0.0/16or2001:db8::/48) - Click "Calculate Subnet" or press Enter to compute results
- Review the output grid — each field shows a specific subnet property
- Click any value to copy it to your clipboard instantly
Common Subnet Reference
| CIDR | Subnet Mask | Total Hosts | Usable Hosts | AWS Usable |
|---|---|---|---|---|
| /16 | 255.255.0.0 | 65,536 | 65,534 | 65,531 |
| /20 | 255.255.240.0 | 4,096 | 4,094 | 4,091 |
| /21 | 255.255.248.0 | 2,048 | 2,046 | 2,043 |
| /22 | 255.255.252.0 | 1,024 | 1,022 | 1,019 |
| /23 | 255.255.254.0 | 512 | 510 | 507 |
| /24 | 255.255.255.0 | 256 | 254 | 251 |
| /25 | 255.255.255.128 | 128 | 126 | 123 |
| /26 | 255.255.255.192 | 64 | 62 | 59 |
| /27 | 255.255.255.224 | 32 | 30 | 27 |
| /28 | 255.255.255.240 | 16 | 14 | 11 |
| /29 | 255.255.255.248 | 8 | 6 | 3 |
| /30 | 255.255.255.252 | 4 | 2 | N/A |
Frequently Asked Questions
What does /24 mean in CIDR notation?
A /24 prefix means the first 24 bits of the address identify the network, leaving 8 bits for hosts. This yields 256 total addresses (2⁸) and 254 usable host addresses (subtracting network and broadcast). It corresponds to the subnet mask 255.255.255.0.
How many IPs does AWS reserve per subnet?
AWS reserves 5 IP addresses in every VPC subnet: the network address (.0), the VPC router (.1), the DNS resolver (.2), a reserved address for future use (.3), and the broadcast address (last IP). The minimum usable subnet in AWS is a /28, which provides 11 usable addresses.
Does this calculator support IPv6?
Yes. Enter any IPv6 CIDR such as 2001:db8::/32 and the calculator will return the network address, last address, prefix length, and total address count. IPv6 subnets are astronomically large — a /64 alone contains 18,446,744,073,709,551,616 addresses.
What is the difference between /31 and /30?
A /30 gives 4 addresses (2 usable), traditionally used for point-to-point links. A /31 (RFC 3021) gives only 2 addresses with no network or broadcast — both IPs are usable, making it slightly more efficient for router-to-router links. Many modern network devices support /31 for this purpose.
Is my IP data sent to any server?
No. All calculations run entirely in your browser using JavaScript. No data is transmitted — your IP ranges, network designs, and CIDR inputs never leave your device.
What is a wildcard mask?
A wildcard mask is the bitwise inverse of a subnet mask. It is used in Cisco ACLs and OSPF configurations to specify which bits of an address are significant. For a /24 subnet (mask 255.255.255.0) the wildcard mask is 0.0.0.255.