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Subnet Calculator (CIDR): Standard, Reverse, VLSM and Supernetting in One Tool

Subnetting has four genuinely different problems hiding inside it, working out one subnet’s details, finding the smallest prefix that fits a required host count, splitting one network into several right-sized pieces for different departments and rolling several subnets back up into a single summary route. This tool handles all four in one place. Enter any address and prefix to see its full standard breakdown with a binary bit view, or switch modes to find a minimum prefix from a required host count, allocate a full VLSM calculator table across multiple department requirements without any overlap or summarize several subnets into the smallest CIDR block that covers them all, useful for both CCNA-style subnetting practice and real cloud VPC CIDR planning on AWS, Azure or GCP.

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How to Use

This tool has four tabs: Standard, Reverse, VLSM and Supernet.

Step 1: Using Standard mode

• Enter your IP Address.
• Select Mask Input, either CIDR prefix (pick from /8 up to /32 in the dropdown) or Custom mask if you want to type a dotted-decimal mask like 255.255.255.192 directly, which the tool automatically converts to and from CIDR notation for you.
• Tap Calculate Subnet. The result shows the network address, broadcast address, first and last usable host, total usable host count, the subnet mask and wildcard mask and a binary bit visualization shading network bits against host bits across all four octets, so you can see exactly where the boundary actually falls.

Step 2: Using Reverse mode

• Switch to the Reverse tab and enter your Required Usable Hosts, the number of devices this subnet actually needs to support.
• Optionally enter a Base Network address to see the resulting range applied to a real network.
• Tap Find Minimum Prefix. The tool finds the smallest possible prefix, meaning the most address-efficient subnet, whose usable host count still meets or exceeds your requirement , and shows you exactly why one bit fewer would not have been enough.

Step 3: Using VLSM mode

• Switch to the VLSM tab and enter your Base Network address and Base Prefix, the block you’re dividing up.
• Tap Add Subnet for each department, floor, or VLAN you need to allocate space for and enter a name and required host count for each one.
• Tap Allocate VLSM. The tool sorts your requirements largest-first and allocates each one to its own size-aligned boundary, so subnets never overlap, and shows a full allocation table with each subnet’s network, mask, usable range, broadcast and spare capacity. If your base block is too small to fit every requirement, it tells you clearly instead of allocating something invalid.

Step 4: Using Supernet mode

• Switch to the Supernet tab and paste your list of subnets in CIDR notation, one per line, into the box, at least two are needed.
• Tap Summarize Routes. The tool finds the smallest single CIDR block that covers every subnet in your list, useful for reducing the number of entries in a routing table.

Step 5: Exporting your result

• Once you have a result in any tab, use Download CSV for the VLSM allocation table or the standard breakdown, Print / PDF for a clean printable copy or Copy to paste the figures elsewhere.

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Key Features

• Standard subnet breakdown with network, broadcast, usable range, wildcard mask and a full binary bit visualization across all four octets
• Custom mask input that auto-converts freely between dotted-decimal masks and CIDR notation
• Reverse mode finding the smallest, most address-efficient prefix for a required host count
• VLSM allocation across multiple named subnet requirements, largest-first and boundary-aligned so allocations never overlap, with a clear error if the base block is too small
• Supernetting / route summarization mode finding the smallest CIDR block covering a full list of subnets
• Runs entirely in your browser, no address you type is ever sent to a server
• CSV export for the VLSM allocation table, useful for network documentation
• Genuinely useful for both CCNA subnet calculator style certification practice and real cloud VPC CIDR planning.

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Formula / Logic Used

Usable Hosts

$$Usable\ Hosts = 2^h – 2, \qquad h = 32 – prefix$$

Two addresses are reserved per subnet, the network address itself and the broadcast address, per RFC 950. A /31 is treated as a valid point-to-point link per RFC 3021 and a /32 is a single host route, so both correctly show zero conventional usable hosts in this tool.

Network and Broadcast Address

$$Network = IP\ \&\ Mask, \qquad Broadcast = Network\ |\ (\sim Mask)$$

Reverse Mode: Minimum Prefix from Required Hosts

The tool searches host-bit counts starting from the smallest, and returns the first one where the usable host formula is satisfied:

$$2^h – 2 \geq Required\ Hosts$$

VLSM Allocation (Largest-First, Boundary-Aligned)

Every subnet requirement is sorted by required host count, largest first. Each one is then assigned the smallest prefix that satisfies its own host requirement, aligned to its own natural address boundary, one after another, so no two allocations ever overlap:

Sort all requirements by required hosts, largest first
For each requirement, in that order:
    find its minimum prefix (same formula as Reverse mode)
    align it to the next available boundary for that size
    if it doesn't fit inside the base block, report an error

Allocating the largest requirement first is what actually minimises total wasted address space across the whole set.

Supernetting (Route Summarization)

For a list of subnets, the tool finds their overall lowest and highest address, then searches downward from /32 for the shortest prefix whose network address, once masked, matches for both that lowest and highest address:

Find the overall lowest and highest address across all listed subnets
Search prefixes from /32 down to /0
Return the first (longest) prefix whose masked network address covers both the lowest and highest address

The tool also flags whether this summary is an exact fit or includes some extra address space beyond just the subnets you listed, since a valid summary route sometimes has to cover a slightly wider range than the exact addresses given.

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Who Should Use This Tool

Networking students and anyone preparing for CCNA, CompTIA Network+ or similar certifications who needs realistic subnetting, VLSM and supernetting practice with instant, verified answers. Also useful for network engineers planning real address allocation, department or VLAN subnetting, firewall ACL wildcard masks or CIDR block sizing for an AWS, Azure or GCP cloud VPC.

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Frequently Asked Questions (FAQs)

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Related Tools

• IP Address Class Finder – check classful ranges and special-use addresses alongside your CIDR planning
• Number Base Converter – for the binary math underlying subnet mask calculations
• Regex Tester & Builder – build a validation pattern for IP addresses used elsewhere in your project

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