Subnet Calculator

Tool Overview

This tool splits a base IP network into smaller subnets using variable-length subnet masking (VLSM). You enter a base IP address and a CIDR prefix (for example 10.0.0.0/24). You then add one or more segments: each segment has a name and a number of hosts needed. The tool allocates a subnet for each segment, sorted from largest to smallest, and shows the network address, broadcast address, usable range, subnet mask, CIDR, and usable host count for each. It shows a bar of how the address space is used and can show config snippets for Cisco IOS and Linux Netplan. If the base network is too small for all segments, it reports that address space is exhausted. An optional AI analysis can give short insights on your plan.

Network planners often need to divide one big network into several smaller ones. Each part needs a different number of hosts. Doing it by hand you must pick a subnet size for each part (a power of two that fits the hosts plus network and broadcast), assign addresses one after another, and work out the mask and range for each. One mistake and addresses overlap or you run out of space. This tool does the allocation for you. You list the segments and how many hosts each needs. It picks the smallest block that fits each, allocates in order (largest first), and shows all the details. It does not decode a single IP and mask; it plans many subnets from one base network.

The tool is for network administrators, students, and anyone planning IP subnets. You need a basic idea of IP addresses and CIDR. You enter the base network, add segments with host counts, and read the results. A first-time user can get started in a few steps.

Background and Concept Explanation

An IPv4 address is 32 bits. A CIDR prefix (for example /24) means the first 24 bits are the network part and the rest are the host part. So /24 gives 2 to the power of (32 minus 24) equals 256 addresses in that subnet. Two of those are reserved: the first is the network address, the last is the broadcast address. So you have 254 usable host addresses. To split a bigger network (for example 10.0.0.0/24) into smaller subnets, you use a longer prefix for each part: /25 gives 128 addresses, /26 gives 64, and so on. Each subnet must start on a boundary that matches its size. Variable-length subnet masking (VLSM) means different subnets can have different sizes (different CIDR values) in the same parent network.

Planning by hand you must decide the size for each segment. For n hosts you need at least n plus 2 addresses (network and broadcast). The smallest block that fits is the next power of two above n plus 2. So 10 hosts need 12, and the next power of two is 16; that segment gets a /28 (32 minus 4 equals 28, because 2 to the 4 is 16). You then assign the first 16 addresses to that subnet, the next block to the next segment, and so on. If you assign in the wrong order you can waste space. This tool sorts segments by host count (largest first), picks the block size for each, and allocates in order. It shows network address, broadcast, usable range, subnet mask, CIDR, total and usable hosts, binary mask, and wildcard mask for each subnet. If the total space needed exceeds the base network size, it stops and reports address space exhausted.

Key Features

• Base network. You enter a base IP address (IPv4, four octets like 10.0.0.0) and a CIDR prefix from 1 to 30 using a slider. The tool checks that the IP is valid. So you define the parent network to split.
• Segments. You add one or more segments. Each segment has a name (for example Management, Workstations) and a host count (how many hosts that subnet must support). You can add or remove segments. Names are limited in length; host count is from 1 up to a maximum per segment. The tool allows up to 50 segments. So you list the parts of your network and how big each must be.
• VLSM allocation. The tool sorts segments by host count (largest first). For each segment it finds the smallest power-of-two block that fits (host count plus 2 for network and broadcast). It allocates subnets one after another from the start of the base network. Each subnet gets a network address, broadcast address, CIDR, mask, and usable range. So you get a full subnet plan without doing the math yourself.
• Per-subnet details. For each subnet the tool shows network address, usable range (first to last usable IP), broadcast address, subnet mask, CIDR prefix, total hosts, usable hosts (total minus 2), wildcard mask, and binary mask. You can copy the network/CIDR line. So you have everything needed to configure devices.
• Address space exhausted. If the base network does not have enough addresses for all segments, the tool stops at the segment that does not fit and shows an error message. It still shows the subnets it could allocate. So you know you need a larger base or fewer segments.
• Allocation bar. A horizontal bar shows each subnet as a block; the width is proportional to its size. If there is unused space at the end, it is labeled free space. So you see how the base network is used.
• Config templates. You can switch to a tab that shows sample config snippets for each subnet: Cisco IOS (interface and IP address) and Linux Netplan (addresses with CIDR). You can copy each snippet. So you can paste into routers or servers.
• Optional AI analysis. After a plan is calculated, you can click a button to get short text insights about your network plan. The feature may be unavailable; if so, a message is shown. So you can get extra commentary when the service is available.

Common Use Cases

• Office network. Base network 10.0.0.0/24. Add segments: Management 10 hosts, Workstations 50, Servers 20. The tool allocates subnets (largest first), shows each subnet’s network, range, mask, and usable hosts. Use the details to set up VLANs or routers.
• Check if a base is big enough. Enter your base IP and CIDR. Add all segments with their host counts. If you see address space exhausted, the base is too small; use a larger base (for example /23 instead of /24) or reduce segments.
• Copy configs. After the plan is calculated, open the Config Templates tab. Copy the Cisco or Netplan snippet for a subnet and paste into your device config.
• Teach VLSM. Change the base or add and remove segments and watch how the allocation and bar change. Use the per-subnet CIDR and mask to see how block size relates to prefix length.
• Get AI commentary. When a plan is shown, click the button to request AI analysis. If available, read the short insights; use them as one input, not as the only source of truth.

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

1. Open the tool. You see a base network section and a segments section. Enter a base IP address (for example 10.0.0.0 or 192.168.1.0). The tool checks the format; if invalid, it shows an error. Set the CIDR prefix with the slider (1 to 30). For example /24.
2. Add segments. You start with a few default segments (name and host count). Click the plus button to add more. For each segment, type a name (e.g. Management) and a host count (e.g. 10). You can remove a segment with the X. You can have up to 50 segments. Host count per segment is from 1 up to the allowed maximum.
3. Look at the results. When the base IP and CIDR are valid and you have at least one segment, the tool calculates the subnet plan. If address space is exhausted, an error appears and you see the subnets that did fit. Otherwise you see the allocation bar and the list of subnets.
4. Read each subnet. In the Network Details tab, each subnet shows network address, usable range, broadcast address, mask, CIDR, usable hosts, wildcard mask, and binary mask. Use Copy to copy the network/CIDR line.
5. Switch to Config Templates. You see Cisco IOS and Linux Netplan snippets for each subnet. Copy the snippet you need and paste into your device.
6. Optional: click the button to get AI network analysis. Wait for the response. If the service is unavailable, a short message appears. Use the insight as one reference only.
7. To change the plan, edit the base IP or CIDR, or add, remove, or edit segments. The plan updates when inputs are valid.

Calculations and Logic

The base network must be a valid IPv4 address. The base CIDR must be between 1 and 30. Total address space in the base is 2 to the power of (32 minus base CIDR). Segments are sorted by host count descending (largest first). For each segment, the tool computes the smallest power of two that is at least (host count plus 2). That power determines the block size (2 to that power) and the subnet CIDR (32 minus that power). The first subnet starts at the base network address (as an integer). The next starts at the previous start plus the previous block size, and so on. If for a segment (current offset plus block size) is greater than the total base space, the tool returns an error and the list of subnets allocated so far.

For each allocated subnet: network address is base plus offset (as IP); broadcast is network plus block size minus 1; usable range is network plus 1 to broadcast minus 1; subnet mask is derived from the subnet CIDR (32 minus CIDR zero bits); wildcard mask is the inverse of the subnet mask; binary mask is the subnet mask in binary with dots between octets. Usable hosts equals block size minus 2. The allocation bar uses the same block sizes; total length is the base space; each subnet is a segment of the bar; remaining space is labeled free space.

Assumptions: IPv4 only; base network is the first address of the block (no check that it is actually a network address); segments are allocated in order from the start of the base; no overlap or gap handling beyond sequential allocation.

Tips, Limitations and Best Practices

Use a base IP that is the start of your block (e.g. 10.0.0.0 for a /24). Set the base CIDR to the size of the block you are allowed to split. Add segments in any order; the tool sorts by host count. Give each segment a clear name so the details and configs are easy to match. If you see address space exhausted, increase the base size (e.g. /23) or reduce host counts or remove segments. Use the config templates as a starting point; adjust interface names and other settings for your gear.

The tool supports IPv4 only, not IPv6. CIDR is limited to 1 through 30; /31 and /32 are not used. Maximum 50 segments; host count per segment has an upper limit. The tool does not decode a single IP and mask to show network and broadcast; it only plans multiple subnets from one base. The AI analysis is optional and may be unavailable. Results are not saved; refresh clears the page. For critical networks, verify addresses and masks with another source or device.

If the plan looks wrong, check that the base IP is valid and the base CIDR matches your actual block. Ensure host counts are correct (each segment needs at least 1). Use the allocation bar to see if a lot of space is left or if it is full. If you need more segments, add them and see if address space is still enough.