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CIDR Calculator

Turn a CIDR prefix into a mask, a host count and a usable range.

  • Free
  • No signup
  • Nothing uploaded
  • No watermark

Network

192.168.1.128

Broadcast

192.168.1.191

Usable hosts

62

Subnet mask

255.255.255.192

Address
192.168.1.130
Prefix
/26
Host range
192.168.1.129 – 192.168.1.190
Wildcard mask
0.0.0.63used by Cisco ACLs — the inverse of the subnet mask
Total addresses
64
Address class
Chistorical — classless routing replaced this in 1993
Scope
Private (RFC 1918) — the usual home router range
Address in binary
11000000.10101000.00000001.10000010
Mask in binary
11111111.11111111.11111111.11000000

How to Use CIDR Notation to Size a Network

CIDR Calculator: before and after TEXT A YOUR INPUT CIDRCALCULATOR DONE A RESULT

CIDR notation packs a whole network into one short string. 10.20.0.0/16 says where a block starts and how big it is, but turning /16 into a subnet mask, a host count and an address range in your head takes practice.

Type any address with a slash and a prefix length and this calculator unpacks it: the mask, the wildcard, the first and last usable address, the total count, and the block in binary. You can also go the other way, typing an address and a mask to get the prefix.

No software to install Free Works on any device

Steps to Decode a CIDR Block

  1. 1. Type the CIDR block

    Enter an address, a slash and a number from 0 to 32, such as 172.16.5.4/20. A bare address with no prefix is read as a single /32 host.

  2. 2. Or convert from a mask

    Enter the address followed by a dotted mask, like 10.1.1.1 255.255.252.0, and the Prefix row tells you it is a /22.

  3. 3. Read the range

    The Host range row gives the first and last usable address, and Total addresses gives the full size of the block.

  4. 4. Carve smaller blocks

    Use the split menu to divide the CIDR range into equal smaller prefixes, each listed with its own range.

Try it now

What Is CIDR Notation?

CIDR stands for Classless Inter-Domain Routing, introduced in 1993 to replace the old fixed class A, B and C networks. The number after the slash counts how many leading bits of the 32-bit address identify the network. A /24 fixes 24 bits and leaves 8 for hosts, which gives 256 addresses.

Cloud platforms such as AWS, Azure and Google Cloud, firewall rules and routing tables all describe address ranges this way.

Why Use IP and Network Calculators?

Subnet masks, CIDR ranges and download times all come down to binary maths that is easy to slip on. These calculators do the sums in the page, so you can plan an address range or check a speed claim with confidence.

“One wrong subnet mask can take a whole office offline.”
  • Subnets worked out instantly
  • CIDR ranges made clear
  • Download times estimated
  • Browser details revealed
  • Usable host counts shown
  • IPv4 and IPv6 conversions
  • MAC addresses checked
  • No commands to install

Popular Uses for Subnet Calculators

Network jobs at home and at work often start with one of these sums.

  • Planning an office network Split an address block into subnets for each floor, team or guest WiFi.
  • Reading CIDR notation See the first address, last address and host count behind a range like /24.
  • Download estimates Work out how long a large file will take on a given connection speed.
  • Speed claims Turn megabits per second into megabytes to see what a plan really delivers.
  • Device lookups Find the maker of a network card from its MAC address prefix.
  • Browser checks See your user agent and browser details when a support team asks.
  • WebSocket tests Connect to a WebSocket endpoint and watch messages go back and forth.
  • IPv6 planning Convert an IPv4 address into IPv6 form while preparing a migration.

What the CIDR Calculator Tells You

Prefix to mask and back

See the dotted subnet mask for any prefix, or type a mask and get its prefix length.

Exact block size

Total addresses and usable hosts are both shown, with the special cases of /31 and /32 counted correctly.

Binary view

The address and the mask are written out in binary, which makes it obvious where the network bits stop.

Equal-size splits

Pick a longer prefix and the block is divided, showing how many subnets you get and how many addresses each holds.

Why Use ToolXkit for CIDR Calculator?

Your files stay with you

Everything runs inside your browser. Nothing you open or type is uploaded to a server.

Network Tools for Different Users

Network administrators

Plan subnets, check CIDR ranges and size links for expected bandwidth.

Networking students

Practise subnetting for CCNA and other exams, and check answers quickly.

Developers

Test WebSocket servers, parse user agents and inspect TCP header fields.

IT support staff

Collect browser details and estimate transfer times while helping users.

Small office owners

Understand router settings and IP ranges without calling a technician every time.

Streamers and gamers

Check upload speed, latency and packet loss figures before a stream.

Best Times to Work Out a Subnet

Network calculators help when you are:

  • Setting up a new router
  • Studying for a networking exam
  • Adding a guest WiFi network
  • Estimating a large upload
  • Comparing broadband plans
  • Identifying an unknown device
  • Moving to IPv6
  • Debugging a WebSocket connection

Tips for Working With CIDR

  1. Each step down in prefix doubles the block. A /23 is two /24s, and a /22 is four.

  2. When planning a cloud VPC, start with a roomy block like a /16 and split it into /24 subnets. Resizing later is painful.

  3. Firewall rules want the network address. If you type 10.0.3.77/24, use the Network box value, 10.0.3.0/24, in the rule.

  4. Avoid overlapping ranges between sites you plan to connect by VPN. Check each block's range before choosing.

  5. Remember that 0.0.0.0/0 means every IPv4 address. In a firewall rule it opens something to the whole internet.

Frequently Asked Questions

Answers to common questions about this tool.

What does /24 mean in an IP address?

It means the first 24 bits are the network part, which is the subnet mask 255.255.255.0. The remaining 8 bits give 256 addresses, of which 254 can be assigned to devices.

How do I convert a subnet mask to CIDR?

Count the one bits in the mask. The calculator does it for you: type an address, a space and the mask, and the Prefix row shows the result. For example, 255.255.255.192 is /26.

What CIDR block should I use for an AWS VPC?

AWS allows VPC blocks from /16 to /28. Many teams choose a /16 from a private range, such as 10.0.0.0/16, then split it into /24 subnets for each availability zone. Type it here to see the ranges.

Why does a /31 show two usable addresses?

Under RFC 3021, a /31 used on a point-to-point link has no network or broadcast address, so both addresses are usable. The calculator follows that rule rather than reporting zero.

Is anything sent over the network when I use this?

No. It is pure arithmetic done in your browser. No address is looked up and no packet is sent, so it is safe to type internal ranges from work.

Is CIDR Calculator accurate enough for real network work?

The maths behind addresses and ranges is exact, so the results match what a router expects. Double-check the input, especially the prefix length, since one wrong digit changes the whole range.

Do the calculators scan my network?

No. The subnet and speed calculators only work with the numbers you type in. They send no traffic into your network, and the only tool that connects anywhere is the WebSocket Tester, to the address you enter.

Why is my real download slower than the estimate?

The calculator assumes the full connection speed is available. In practice, WiFi, other devices, server limits and protocol overhead eat into it, so real downloads often take somewhat longer.

Is CIDR Calculator free and private?

Yes. It costs nothing and needs no account. What you type stays in your browser and is never sent to ToolXkit, which matters when it describes a real company network.

What is the difference between Mbps and MBps?

Mbps means megabits per second, the unit broadband plans use. MBps means megabytes per second, the unit most download windows show. One byte is eight bits, so 100 Mbps is about 12.5 MBps.

Further reading

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