Tool Overview

This tool helps you split a single IP address block into smaller subnets. You enter one base block in CIDR form (for example 10.0.0.0/16) and add one or more subnet requirements by saying how many hosts each subnet must support. The tool then computes each subnet’s CIDR, first and last usable IP, broadcast address, gateway, and host counts. It shows whether the subnets fit inside the base block and lets you export the plan or use an optional AI step to suggest subnets from a short description.

Planning subnets by hand is error-prone. You must align each subnet to valid boundaries, reserve the network and broadcast addresses, and avoid overlaps. Mistakes lead to wrong configs or wasted space. This tool does the alignment and math for you so you get valid subnets and a clear view of how much of the block is used.

The tool is for anyone who plans or manages IPv4 networks: developers, sysadmins, and network engineers. You can use it with basic knowledge of IP addresses and CIDR. Understanding what a subnet mask and a gateway are helps, but the tool explains the main outputs.

Background & Concept Explanation

CIDR stands for Classless Inter-Domain Routing. A CIDR block is written as an IP address followed by a slash and a number, for example 192.168.1.0/24. The number is the prefix length: how many bits are fixed for the network part. The rest of the bits identify hosts inside that network. A /24 block has 2 to the power of (32 minus 24) addresses, that is 256. Two of them are reserved: the first is the network address and the last is the broadcast address. So you have 254 usable host addresses in a /24. A related operation involves calculating aspect ratios as part of a similar workflow.

When you split a large block into subnets, each subnet must start at a valid boundary. The size of each subnet must be a power of two. The tool takes your base block and a list of “how many hosts per subnet.” For each requirement it finds the smallest subnet that fits that many hosts (plus the two reserved addresses), places it at the next free boundary inside the base block, and shows you the resulting CIDR, IP range, broadcast, and gateway. Subnets are placed one after another so they do not overlap.

Doing this by hand means a lot of binary or decimal math and easy mistakes. The tool does the math and keeps everything aligned. It only works with IPv4. It does not convert or plan IPv6.

Key Features

• Base CIDR input. You enter one IPv4 block in CIDR notation, for example 10.0.0.0/16. The tool checks that the format is valid (four octets and a prefix length from 0 to 32). If it is invalid you see an error and the plan is not computed. This block is the total space you can split into subnets.
• Subnet requirements. You add one or more subnets. For each subnet you give a name (optional) and the number of hosts it must support. The tool limits how many subnets you can add and the maximum host count per subnet so the page stays fast. Each requirement is converted into a subnet size that fits that many hosts plus the reserved network and broadcast addresses.
• Automatic allocation. Subnets are placed one after another inside the base block, aligned to valid network boundaries. You see each subnet’s CIDR, prefix length, first usable IP, last usable IP, broadcast address, and gateway (the first usable IP). The tool marks a subnet as invalid if it would exceed the base block.
• Plan overview. When the base CIDR is valid you see total IPs in the block, how many are used by your subnets, and how many are left. If the sum of subnet sizes is larger than the base block you get an overflow warning. This helps you stay within the block or add a bigger base.
• Address space bar. A bar shows how much of the base block each subnet uses. Each segment is one subnet. You can see at a glance how full the block is and which subnet uses how much. If the plan overflows, a message explains that the plan exceeds the parent block.
• Show or hide details. Each subnet can show or hide extra details: start IP, end IP, broadcast, gateway, and total IPs. Copy buttons let you copy the CIDR or IPs so you can paste them into configs or docs.
• Export and print. You can export the full plan and requirements as a JSON file for backup or use elsewhere. You can also print the page to get a paper or PDF copy of the configuration.
• AI Planner (optional). You can describe your setup in words (for example “web tier, app tier, database”). The tool sends that text to a backend service which returns suggested subnets (name and host count). Those suggestions replace your current list. This step needs network access and can fail; the tool shows an error if the AI does not return valid data. You can still add and edit subnets manually without using the AI.

Common Use Cases

Planning a small office or VPC. You have one block (e.g. 10.0.0.0/16) and need separate subnets for web servers, app servers, and databases. You add three subnets with the right host counts, name them, and read off the CIDR and gateway for each. You then use those values in your router or cloud console. For adjacent tasks, calculating business days addresses a complementary step.

Checking if a plan fits. You already have a list of subnet sizes. You enter your base CIDR and add one requirement per subnet with the desired host count. The tool shows whether all subnets fit and how much space is left. If it overflows you reduce host counts or use a larger base block.

Documenting an allocation. After you get a plan you export the JSON or print the page. You keep the file or printout so your team knows which CIDR and gateway belong to which subnet.

Getting a first draft with AI. You are not sure how to size subnets. You open the AI Planner, type a short description of your tiers or environments, and click generate. The tool fills in subnet names and host counts. You can then adjust numbers or names and see the updated plan. When working with related formats, calculating subnet masks can be a useful part of the process.

Teaching or learning subnetting. You change the base CIDR and add or remove subnets and watch how the CIDRs, ranges, and capacity bar update. The optional details view shows start IP, end IP, broadcast, and gateway so you can see how the math works in practice.

How to Use This Tool (Step-by-Step)

1. Open the tool. You will see a field for the base network block and an area for subnet requirements.
2. Enter the base CIDR. Type your IPv4 block in CIDR form, for example 10.0.0.0/16 or 192.168.0.0/24. Use the format “address/prefix”. The tool checks the format; when it is valid the field is marked and the plan area becomes active.
3. Add subnets. Click “Add Subnet” to add a row. For each subnet enter an optional name (e.g. “Web Tier”) and the number of hosts it must support. The tool will choose the smallest subnet size that fits that many hosts plus the reserved addresses. You can add several subnets; there is a maximum number of subnets per plan.
4. Review the plan. Each subnet row shows the computed CIDR, prefix length, and usable host count. If the base CIDR is valid you also see a plan overview (total IPs, used, available) and a usage bar. Subnets that exceed the base block are marked invalid.
5. Show details if needed. Use the “Show Details” control to expand each subnet and see start IP, end IP, broadcast, and gateway. Use the copy buttons next to CIDR or IPs to copy values for use elsewhere.
6. Use the AI Planner if you want. Click “AI Planner” and type a short description of your network (e.g. “VPC with web, app, and database”). Click generate. The tool replaces your current subnet list with the AI suggestions. You can then edit names or host counts. If the AI fails you will see an error; you can still add subnets manually.
7. Export or print. When the plan looks good use “Export JSON” to download the plan and requirements. Use “Print Config” to print the page for a paper or PDF record.

Calculations & Logic

The tool works only with IPv4. It converts each IP to a 32-bit number so it can do arithmetic. The base CIDR is parsed into a network address and a size. The network address is the IP with all host bits set to zero (using the prefix length). The size is two to the power of (32 minus the prefix length). For example /24 gives size 256.

For each subnet requirement the tool finds the smallest prefix length that can hold that many hosts plus two (network and broadcast). For example 100 hosts need 102 addresses, so the next power of two is 128, which is 2^7, so the prefix is 32 minus 7 = 25. The subnet is a /25. The tool then places that subnet at the next free address in the base block, aligned so the subnet starts at a multiple of its size. Each following subnet starts right after the previous one. In some workflows, checking your IP address is a relevant follow-up operation.

Inside each subnet the first address is the network address, the last is the broadcast, and the rest are usable. The tool shows the first usable IP as the gateway (a common choice). Usable host count is the subnet size minus two. Total capacity of the plan is the size of the base block. Used capacity is the sum of all subnet sizes. If used is greater than total, the plan is marked as overflowing.

Reference: Prefix Length and Subnet Size (IPv4)

Larger prefixes (e.g. /16) mean larger subnets (e.g. /16 is 65536 IPs, 65534 usable). The tool picks the prefix from the host count you ask for; the table above shows common sizes.

Tips, Limitations & Best Practices

Only IPv4 is supported. Do not enter IPv6 addresses or prefixes; they will be rejected or give wrong results. For related processing needs, looking up IP addresses handles a complementary task.

Enter the base CIDR in the form “x.x.x.x/n” with no spaces. The address must be four numbers between 0 and 255 separated by dots. The prefix must be between 0 and 32. Leading zeros in an octet (e.g. 10.00.0.0) are not accepted.

Host count is the number of hosts you need in that subnet. The tool reserves the network and broadcast addresses and chooses a subnet big enough to fit the rest. If you need 50 hosts you get a subnet that can hold at least 52 addresses (50 plus 2 reserved), so a /26 (64 addresses, 62 usable) is chosen.

There is a maximum number of subnets per plan and a maximum host count per subnet. If you hit the subnet limit you cannot add more until you remove one. Large host counts are clamped so the tool stays responsive.

The AI Planner is optional and depends on a remote service. If it fails you see an error; your manually entered subnets are unchanged. You can always add and edit subnets without using the AI.

Export saves the current plan and requirements plus a timestamp. It does not include passwords or secrets. Keep the JSON file in a safe place if you use it for real network config.

Subnets are allocated in the order you list them. If you need a specific order (e.g. a DMZ first), add the subnets in that order. You can remove and re-add to change the order.