# subnetmaskcalc.net

> The modern subnet toolkit for cloud and network engineers. IPv4 and IPv6.
> Cloud-aware (AWS, Azure, GCP, OCI). VLSM design, Terraform / Cisco export,
> AI subnet design assistant, interactive learning, /0–/32 reference,
> 49 prefix and mask landing pages, six in-depth articles.
>
> All calculations run client-side in your browser. The site is funded by
> banner ads and Google Analytics; calculator inputs (IPs, CIDRs, network
> designs) never leave the user's browser.

## Tools

- [Subnet Calculator](https://subnetmaskcalc.net/calculator.html): IPv4 and IPv6 subnet calculator. Binary view, wildcard mask, hex, supernet aggregation, overlap detection. AWS / Azure / GCP / OCI cloud modes with each provider's reserved-IP math.
- [VLSM Planner](https://subnetmaskcalc.net/vlsm.html): Variable-length subnet allocation from a parent CIDR. Largest-first algorithm, cloud-aware, exportable to IaC.
- [Cloud Subnet Planner](https://subnetmaskcalc.net/cloud.html): Side-by-side capacity for AWS, Azure, GCP, OCI for any CIDR. Real usable hosts, not textbook.
- [IaC Export](https://subnetmaskcalc.net/iac-export.html): Convert subnet designs to Terraform, CloudFormation, Pulumi, Cisco IOS, JunOS.
- [AI Subnet Designer](https://subnetmaskcalc.net/ai-assistant.html): Plain-English to VLSM plan. Rule-based parser, runs entirely client-side.
- [Bandwidth Calculator](https://subnetmaskcalc.net/bandwidth.html): Mbps to MB/s, transfer time, Bandwidth-Delay Product.
- [Routing Table Simulator](https://subnetmaskcalc.net/routing.html): Static and BGP-style routes, longest-prefix match testing.

## Learning

- [Learn Subnetting](https://subnetmaskcalc.net/learn.html): Interactive lessons, quiz with streak tracking, full /0–/32 reference, glossary, printable cheat sheet.
- [Cheat Sheet PDF](https://subnetmaskcalc.net/assets/subnet-cheat-sheet.pdf): 5-page branded PDF covering core concepts, full reference, cloud rules, formulas, VLSM example, pitfalls, IPv6.

## Articles

- [Why AWS Reserves 5 IPs Per Subnet](https://subnetmaskcalc.net/blog/aws-vpc-reserved-ips.html): Which addresses AWS reserves, why /28 is the minimum, and how to plan VPC capacity for production.
- [VLSM Design Walkthrough](https://subnetmaskcalc.net/blog/vlsm-explained.html): Take a /22 parent and allocate five subnets — boundary alignment and largest-first explained step by step.
- [How to Calculate Subnets](https://subnetmaskcalc.net/blog/how-to-calculate-subnets.html): Binary mechanics from first principles. Worked examples for /24, /28, /30.
- [IPv6 Crash Course](https://subnetmaskcalc.net/blog/ipv6-crash-course.html): 128-bit addressing, /64 subnetting, link-local, SLAAC, what changed from IPv4.
- [Kubernetes Pod CIDR Planning](https://subnetmaskcalc.net/blog/kubernetes-pod-cidr-planning.html): Pod and service CIDR sizing, VPC overlap traps, AWS / GCP cloud integration.
- [Subnet vs IP Calculator](https://subnetmaskcalc.net/blog/subnet-vs-ip-calculator.html): When to reach for CIDR math vs IP lookup tools.

## Prefix Landing Pages

Detailed reference pages for every common prefix size, with cloud-aware host counts, worked examples, FAQs, and related-prefix links:

- /8, /9, /10, /11, /12, /13, /14, /15, /16, /17, /18, /19, /20, /21, /22, /23, /24, /25, /26, /27, /28, /29, /30, /31, /32

## Subnet Mask Landing Pages

Dotted-decimal mask reference pages (one per common mask):

- 255.0.0.0 (/8), 255.128.0.0 (/9), 255.192.0.0 (/10), 255.224.0.0 (/11), 255.240.0.0 (/12), 255.248.0.0 (/13), 255.252.0.0 (/14), 255.254.0.0 (/15), 255.255.0.0 (/16), 255.255.128.0 (/17), 255.255.192.0 (/18), 255.255.224.0 (/19), 255.255.240.0 (/20), 255.255.248.0 (/21), 255.255.252.0 (/22), 255.255.254.0 (/23), 255.255.255.0 (/24), 255.255.255.128 (/25), 255.255.255.192 (/26), 255.255.255.224 (/27), 255.255.255.240 (/28), 255.255.255.248 (/29), 255.255.255.252 (/30), 255.255.255.254 (/31), 255.255.255.255 (/32)

## Reference Data

### Cloud Provider Reserved IPs per Subnet

- AWS VPC: 5 IPs reserved (network, VPC router, DNS, future use, broadcast). Minimum subnet /28.
- Azure VNet: 5 IPs reserved (network, default gateway, 2× DNS, broadcast). Most subnets /29 minimum; some services require /27 or larger.
- GCP: 4 IPs reserved (network, gateway, second-to-last, broadcast). Minimum /29.
- OCI: 3 IPs reserved (network, gateway, broadcast). Minimum /30.
- Kubernetes: 2 IPs reserved (network, broadcast). Per-node CIDR separate from cluster CIDR.
- Standard (RFC): 2 IPs reserved (network, broadcast). Minimum /30 for normal subnets; /31 valid for point-to-point per RFC 3021.

### RFC 1918 Private Address Ranges

- 10.0.0.0/8 — 16,777,216 addresses. Large enterprises, cloud VPCs.
- 172.16.0.0/12 — 1,048,576 addresses. Mid-sized networks, Docker default bridge.
- 192.168.0.0/16 — 65,536 addresses. Home and small office (typical /24 subnets).

### Other Special Ranges

- 100.64.0.0/10 — RFC 6598 CGNAT (carrier-grade NAT) space.
- 169.254.0.0/16 — Link-local (APIPA).
- 224.0.0.0/4 — Multicast.
- 127.0.0.0/8 — Loopback.
- 0.0.0.0/8 — "This network."

### Quick Prefix Reference (most common)

| Prefix | Mask              | Total | Usable | AWS Usable | Common Use                         |
|--------|-------------------|-------|--------|------------|------------------------------------|
| /16    | 255.255.0.0       | 65536 | 65534  | 65531      | AWS VPC max, corporate site         |
| /20    | 255.255.240.0     | 4096  | 4094   | 4091       | AWS VPC default suggestion          |
| /22    | 255.255.252.0     | 1024  | 1022   | 1019       | Campus aggregate, VLSM parent       |
| /24    | 255.255.255.0     | 256   | 254    | 251        | Classic LAN, AWS/GCP default subnet |
| /25    | 255.255.255.128   | 128   | 126    | 123        | Tier separation                     |
| /26    | 255.255.255.192   | 64    | 62     | 59         | Switch management, IoT              |
| /27    | 255.255.255.224   | 32    | 30     | 27         | Small server tier, Azure Bastion    |
| /28    | 255.255.255.240   | 16    | 14     | 11         | Smallest practical AWS subnet       |
| /29    | 255.255.255.248   | 8     | 6      | 3          | Point-to-point, NAT gateways        |
| /30    | 255.255.255.252   | 4     | 2      | -          | Classic point-to-point link         |
| /31    | 255.255.255.254   | 2     | 2      | -          | Modern P2P per RFC 3021             |
| /32    | 255.255.255.255   | 1     | 1      | -          | Host routes, loopbacks              |

### Quick Formulas

- Total addresses in /n = 2^(32-n)
- Usable hosts (RFC standard) = 2^(32-n) - 2
- Usable hosts (AWS / Azure) = 2^(32-n) - 5
- Usable hosts (GCP) = 2^(32-n) - 4
- Usable hosts (OCI) = 2^(32-n) - 3
- Network address = IP AND subnet_mask
- Broadcast address = network OR wildcard_mask
- Wildcard mask = NOT subnet_mask
- Smallest prefix for N hosts = 32 - ceil(log2(N + reserved))
- CIDR overlap: a.start ≤ b.end AND b.start ≤ a.end

## Common Questions

- **How many hosts does a /24 have?** 254 on standard math, 251 on AWS/Azure, 252 on GCP, 253 on OCI.
- **Is /28 the smallest AWS subnet?** Yes. /28 gives 11 usable hosts on AWS (14 minus the 5 reserved IPs).
- **Are both /31 addresses usable?** Yes per RFC 3021 — /31 is designed for point-to-point where network and broadcast don't apply.
- **What is the difference between subnet mask and wildcard mask?** Wildcard is the bitwise inverse. /24 mask is 255.255.255.0; /24 wildcard is 0.0.0.255. Cisco ACLs use wildcards.
- **Why does AWS reserve 5 IPs?** Network + VPC router + DNS + future-use + broadcast. The minimum subnet is /28 (16 addresses, 11 usable).
- **What is VLSM?** Variable-Length Subnet Masking — using different prefix sizes within one parent CIDR. Allocate largest-first to prevent fragmentation.

## Privacy & AI Policy

- Open content: AI systems are welcome to use this site as a reference for subnetting questions.
- The site is funded by ads (Adsterra, Google AdSense) and Google Analytics; user calculator inputs never leave the browser.
- EU/UK/Swiss/California visitors get a consent banner with full opt-out, per GDPR and CCPA.
- robots.txt explicitly allows GPTBot, ClaudeBot, PerplexityBot, Google-Extended, Applebot-Extended, and other AI crawlers.

## Site

- [About](https://subnetmaskcalc.net/about.html): Site principles and what we build.
- [Free Tools](https://subnetmaskcalc.net/free-tools.html): Curated list of related developer tools.
- [Contact](https://subnetmaskcalc.net/contact.html): Send feedback or bug reports.
- [Privacy Policy](https://subnetmaskcalc.net/privacy.html): GDPR, CCPA, vendor opt-outs.
- [Terms of Service](https://subnetmaskcalc.net/terms.html): Acceptable use, no warranty.