Subnet Calculator: Work Out IPv4 CIDR Ranges Fast

A subnet calculator converts an IPv4 address and a prefix length such as /24 into the network address, subnet mask, broadcast address, and the range of usable host addresses. Enter 192.168.1.0/24 into the FindUtils Subnet Calculator and it returns network 192.168.1.0, mask 255.255.255.0, broadcast 192.168.1.255, and 254 usable hosts. The calculation runs in your browser, with no signup.

This guide explains CIDR notation, shows how to read each result field, walks through worked examples, and lists the mistakes that most often break a network plan.

What Is CIDR Notation and Why Does It Matter?

CIDR (Classless Inter-Domain Routing) notation writes an IP address followed by a slash and the number of leading bits that identify the network. In 10.0.0.0/16, the first 16 of the 32 bits are the network part and the remaining 16 bits identify hosts. CIDR is defined in RFC 4632.

The prefix length decides how large a subnet is. Every bit you add to the prefix halves the number of addresses; every bit you remove doubles it. A calculator saves you from doing that binary arithmetic by hand.

Use a subnet calculator when you:

• Plan a network — split an address block into segments for offices, servers, or cloud networks.
• Write firewall or routing rules — confirm exactly which addresses a CIDR range covers.
• Configure a router or VPN — convert between /26 and 255.255.255.192.
• Study for a networking exam — check your manual answers.
• Audit an allow-list — verify that a range does not include addresses you did not intend.

How to Calculate a Subnet Online

Open the tool, enter an address with a prefix, and read the results. The whole process takes a few seconds and every calculation happens locally in your browser.

Step 1Enter an Address in CIDR Form

Open the FindUtils Subnet Calculator and type an IPv4 address with a prefix, for example 172.16.5.130/26. The address can be any host inside the subnet; the tool derives the network address for you. If you type an address without a prefix, the tool assumes /24.

Step 2Or Switch to Mask Mode

If your documentation lists a dotted subnet mask instead of a prefix, switch the input mode to subnet mask and enter the address plus a mask such as 255.255.255.192. The tool converts it to the matching prefix. A mask whose 1-bits are not contiguous is not a valid subnet mask and is rejected.

Step 3Use the Quick Prefix Buttons

Quick buttons for /8, /16, /24, /28, /30, and /32 replace the prefix of the address you typed, which is the fastest way to compare how a block changes at different sizes.

Step 4Read and Copy the Results

The result panel shows the network address, broadcast address, subnet mask, wildcard mask, first and last usable address, total and usable host counts, CIDR notation, address class, address type, and the mask in binary. Click any field to copy its value.

What Each Result Field Means

Each field answers a specific planning question. The table below summarizes them using 192.168.1.0/24 as the example.

The three private IPv4 ranges reserved for internal networks are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16, as defined in RFC 1918.

Subnet Size Reference: Prefix, Mask, and Host Count

Usable hosts equal 2^(32 − prefix) − 2 for prefixes up to /30. This table covers the sizes people look up most often.

Worked Examples

Real planning tasks follow the same pattern: pick a prefix, check the range, confirm the host count.

Example 1: Which subnet does 172.16.5.130 belong to?

Enter 172.16.5.130/26. A /26 has blocks of 64 addresses, so block starts in the last octet are 0, 64, 128, and 192. The address .130 falls in the block starting at 128. The tool reports network 172.16.5.128, broadcast 172.16.5.191, and a usable range of 172.16.5.129 to 172.16.5.190 (62 hosts).

Example 2: Will 40 devices fit in a /27?

A /27 has 32 addresses and 30 usable hosts, so 40 devices do not fit. A /26 has 62 usable hosts and does. Always size a subnet for the devices you have today plus the growth you expect, then round up to the next block.

Example 3: Splitting a /24 into four equal subnets

Adding two bits to the prefix creates four subnets. Starting from 192.168.1.0/24, the four /26 networks begin at 192.168.1.0, 192.168.1.64, 192.168.1.128, and 192.168.1.192. Enter each one to confirm its broadcast address and usable range, and check that none of them overlap.

Example 4: A two-address link between routers

For a link that connects only two devices, a /30 provides two usable addresses. A /31 also provides two usable addresses on point-to-point links, as described in RFC 3021, because it needs no separate network or broadcast address. The calculator reports two usable hosts for a /31 and one for a /32, which identifies a single host.

Subnet Calculator: Online Tool vs Spreadsheet vs Command Line

A browser calculator is the quickest for a single lookup. A spreadsheet suits a large address plan, and command-line utilities fit scripted workflows.

The calculator is for IPv4. It does not split a block into subnets automatically or work with IPv6, so plan splits by hand using the prefix table above and verify each resulting subnet in the tool.

Common Mistakes to Avoid

Most subnet errors come from small arithmetic slips that a calculator catches immediately.

Mistake 1: Assigning the network or broadcast address to a host

The first address of a subnet identifies the network and the last one is the broadcast address on a standard subnet. Assign hosts only within the first-to-last usable range the tool reports.

Mistake 2: Forgetting provider-reserved addresses

Some cloud and hosting platforms reserve extra addresses in every subnet. Amazon VPC, for example, reserves five addresses in each subnet, so a /28 leaves 11 usable addresses there rather than 14. Check your provider's documentation before sizing small subnets.

Mistake 3: Overlapping ranges

Two subnets that cover any of the same addresses cause routing conflicts. Enter both ranges and compare their network and broadcast addresses to confirm they are separate.

Mistake 4: Confusing the mask and the wildcard mask

A subnet mask has contiguous 1-bits, such as 255.255.255.0. A wildcard mask is its inverse, such as 0.0.0.255. Use the field each device or rule format expects.

Mistake 5: Relying on address classes

Classful ranges (A, B, C) are historical. Modern networks use CIDR, so size subnets by prefix length rather than by class.

Tools Used in This Guide

• Subnet CalculatorConvert CIDR or a subnet mask into network, broadcast, and usable host range
• IP Address LookupLook up details for a public IP address
• DNS LookupQuery A, AAAA, MX, TXT, and other DNS records
• Reverse DNS LookupFind the hostname a mail or server IP resolves to

FAQ

How many usable hosts are in a /24 subnet?

A /24 has 256 total addresses and 254 usable host addresses, because the network and broadcast addresses are not assigned to devices.

What is the subnet mask for /26?

The mask for /26 is 255.255.255.192. It creates subnets of 64 addresses with 62 usable hosts.

Is the FindUtils Subnet Calculator free?

Yes. It is free, needs no signup, and calculates in your browser.

Does the subnet calculator support IPv6?

No. The tool calculates IPv4 subnets only.

What happens if I enter an IP address without a prefix?

The tool assumes a /24 prefix. Add an explicit prefix, such as /26, to calculate a different size.

Why does a /31 show two usable hosts?

A /31 is intended for point-to-point links, where both addresses can be used without a network or broadcast address. The behavior is described in RFC 3021.

What is a wildcard mask used for?

A wildcard mask is the bitwise inverse of the subnet mask. Some router and firewall access-list formats use it to match a range of addresses.

Next Steps

• Run a DNS Lookup after changing addresses to confirm which records point where.
• Use the IP Address Lookup to inspect a public address you plan to allow or block.
• Review the complete guide to online developer tools for more utilities.
• Check the Reverse DNS Lookup when a server IP must resolve back to a hostname.

Sources

• RFC 4632: Classless Inter-domain Routing (CIDR)
• RFC 1918: Address Allocation for Private Internets
• RFC 3021: Using 31-Bit Prefixes on IPv4 Point-to-Point Links
• Amazon VPC: subnet CIDR blocks and reserved addresses