CIDR Split Calculator: Subnet Divider

A CIDR split calculator takes a larger IP network written in CIDR notation (like 192.168.1.0/24) and divides it into smaller, equal-sized subnets, showing you the network address, broadcast address, usable host range, and subnet mask for each piece. You enter the base network and the number of subnets (or the prefix length you want), and the calculator returns a complete breakdown of every resulting subnet. This is the fastest way to plan address space for multiple VLANs, branch offices, or cloud subnets without doing binary math by hand.

Formula

The core calculation is simple once you know the rule: to split a network into N equal subnets, you need to add k bits to the original prefix length, where 2^k = N. The new prefix length is:

new_prefix = original_prefix + log₂(N)

Where:

• original_prefix – the number of fixed bits in the base network (e.g., 24 for a /24).
• N – the number of subnets you want (must be a power of 2 for equal splits).
• new_prefix – the prefix length of each resulting subnet (e.g., /26).

Each subnet then has 2^{(32 – new_prefix)} total addresses, of which 2 are reserved (network and broadcast), leaving 2^{(32 – new_prefix)} – 2 usable hosts. The calculator above assumes IPv4; IPv6 works the same way but with 128 bits instead of 32.

Worked Example

Using the default values: base network 192.168.1.0/24, split into 4 subnets.

1. Original prefix = 24.
2. Number of subnets = 4, so k = log₂(4) = 2.
3. New prefix = 24 + 2 = 26.
4. Total addresses per subnet = 2^(32-26) = 2⁶ = 64.
5. Usable hosts per subnet = 64 – 2 = 62.
6. Total usable hosts across all 4 subnets = 62 × 4 = 248.

The four resulting subnets are:

• 192.168.1.0/26 – usable 192.168.1.1 to 192.168.1.62
• 192.168.1.64/26 – usable 192.168.1.65 to 192.168.1.126
• 192.168.1.128/26 – usable 192.168.1.129 to 192.168.1.190
• 192.168.1.192/26 – usable 192.168.1.193 to 192.168.1.254

Notice the pattern: each subnet starts at a multiple of 64 (the block size), and the broadcast address is the last address before the next subnet’s start.

When to Use a CIDR Split

You reach for a split calculator when you have a block of IP space and need to carve it into smaller, non-overlapping pieces. Common real-world cases:

• Dividing a /24 into four /26s for four departments in an office.
• Splitting a cloud VPC (e.g., a /16) into multiple /24 subnets for different tiers (web, app, database).
• Breaking a large block into equal chunks for separate customer networks.
• Planning a new site’s addressing scheme so each floor gets its own subnet.

The key requirement is that the number of subnets must be a power of 2 (2, 4, 8, 16, etc.) if you want them all the same size. If you need unequal sizes (e.g., one /26 and two /27s), you’re doing VLSM (Variable Length Subnet Masking), which is a different process.

How to Choose the Right Split

Before you split, decide what you actually need:

1. Count the maximum number of devices per subnet. Add a few extra for growth.
2. Find the smallest prefix that gives you that many usable hosts. Use the formula: usable = 2^(32-prefix) – 2.
3. Pick a number of subnets that is a power of 2 and whose prefix length is larger than your base.
4. Check that the total usable hosts across all subnets is enough for your whole network.

For example, if each subnet needs 20 usable hosts, a /27 gives 30 usable (2⁵ – 2 = 30), which fits. A /28 gives only 14, too small. So you’d split your /24 into 8 /27s (since 24 + 3 = 27, and 2³ = 8).

Common Mistakes and Misconceptions

Even experienced engineers slip up on these:

• Forgetting the network and broadcast addresses. A /30 has 4 total addresses but only 2 usable – many people mistakenly think it has 4 usable.
• Assuming you can split into any number of subnets. You cannot split a /24 into 3 equal subnets. You can only split into powers of 2. For 3 subnets, you’d need VLSM or unequal sizes.
• Mixing up prefix length and subnet mask. /24 is not the same as 255.255.255.0? Actually it is – /24 means 24 ones in the mask, which is 255.255.255.0. The mistake is thinking /25 is 255.255.255.128 (it is) but then counting hosts wrong.
• Not accounting for the network address in the first subnet. The first subnet’s network address is the same as the base network’s address, but that’s fine – it’s still a valid subnet.
• Using the calculator for IPv6 without adjusting. IPv6 has 128 bits, and the host count formula changes (no broadcast, but still a subnet-router anycast). This calculator is IPv4-only.

Limits of the Formula

The formula assumes a clean, equal split of a single contiguous block. It does not handle:

• Unequal subnet sizes (VLSM).
• Overlapping subnets – if you split a /24 into two /25s, they don’t overlap, but if you then try to use one of those /25s and also a /26 that overlaps it, you’ll get conflicts.
• Reserved addresses beyond network/broadcast (e.g., some cloud providers reserve the first few IPs in each subnet).
• IPv6 – the bit count and address format are different.

For VLSM, you need a different tool that lets you specify each subnet’s size individually. For cloud providers, always check their documentation for how many addresses they reserve per subnet.

Frequently Asked Questions

Can I split a /24 into 3 subnets?

Not into 3 equal subnets, because 3 is not a power of 2. You can split it into 2 (/25s) or 4 (/26s), or use VLSM to create, say, one /25 and two /26s (which totals 3 subnets of different sizes).

What is the difference between a /26 and a 255.255.255.192 mask?

Nothing – they are two ways of writing the same thing. /26 means 26 bits are set to 1 in the subnet mask, which in decimal is 255.255.255.192. The slash notation is just shorter.

How many usable hosts are in a /28?

A /28 has 2^(32-28) = 16 total addresses, minus 2 for network and broadcast, leaving 14 usable. This is a common size for small point-to-point links or tiny subnets.

Why does my cloud provider show fewer usable IPs than the formula?

Most cloud providers reserve a few IP addresses per subnet for internal use (e.g., the first four and the last one). AWS, for example, reserves 5 IPs per subnet. Always check your provider’s documentation; the formula here gives the theoretical maximum.

Can I split an IPv6 network the same way?

Yes, the math is identical but with 128 bits instead of 32. However, IPv6 subnets are usually /64 or larger, and the host count is so huge that the “usable hosts� concept is rarely a concern. This calculator is IPv4-only.

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in base | Base network (CIDR) | - | 192.168.1.0/24 | 0
in subnets | Number of subnets | - | 4 | 1
out new_prefix | New prefix length | / | 26 | 0
out hosts_per_subnet | Usable hosts per subnet | - | (2^(32-new_prefix))-2 | 0
out total_hosts | Total usable hosts | - | ((2^(32-new_prefix))-2)*subnets | 0