IPv4 Subnet Calculator: CIDR and Host Ranges
Calculate IPv4 subnets from an address and CIDR prefix. Learn network and broadcast addresses, usable host ranges, masks, and the special /31 and /32 rules.
IPv4 subnet calculation uses a CIDR prefix to split a 32-bit address into network and host bits. For a quick answer, enter an IP address and prefix in the IPv4 subnet calculator. It calculates the network address, subnet mask, broadcast address, usable host range, wildcard mask, and address counts locally in your browser.
What the IPv4 Subnet Calculator Returns
Enter a host address such as 192.168.1.10 and a CIDR prefix of 24. The result is:
| Field | Result | Meaning |
|---|---|---|
| CIDR notation | 192.168.1.10/24 | The original IP and prefix |
| Subnet mask | 255.255.255.0 | The first 24 bits are network bits |
| Network address | 192.168.1.0 | The start of the subnet |
| Broadcast address | 192.168.1.255 | The last address in a traditional broadcast subnet |
| Usable host range | 192.168.1.1 – 192.168.1.254 | Addresses normally assignable to hosts |
| Total addresses | 256 | Includes network and broadcast addresses |
| Usable hosts | 254 | Excludes the first and last address in a normal subnet |
| Wildcard mask | 0.0.0.255 | The bitwise inverse of the subnet mask |
This output is useful when checking server interfaces, routes, ACL ranges, or whether two addresses belong to the same subnet. The calculator supports every prefix from /0 through /32 and updates the results as you type.
How CIDR Prefixes Map to Subnet Masks
A CIDR (Classless Inter-Domain Routing) prefix is the number of consecutive network bits. /24 means that the first 24 bits are 1, which is 255.255.255.0 in dotted-decimal notation. Eight host bits remain, so the subnet contains 2⁸ = 256 addresses.
Here are several useful reference points:
| CIDR | Subnet mask | Total addresses | Usable hosts in a normal subnet |
|---|---|---|---|
/8 | 255.0.0.0 | 16,777,216 | 16,777,214 (RFC 4632) |
/16 | 255.255.0.0 | 65,536 | 65,534 (RFC 4632) |
/24 | 255.255.255.0 | 256 | 254 |
/28 | 255.255.255.240 | 16 | 14 |
/30 | 255.255.255.252 | 4 | 2 |
Each time the prefix grows by one, one host bit disappears and the address count is halved. RFC 4632 documents CIDR addressing and aggregation. To inspect how individual binary octets produce their decimal values, use the online number base converter alongside the calculator.
Calculate an IPv4 Subnet Step by Step
Use the calculator's built-in example, 10.20.30.45/20:
- Open the online IPv4 subnet calculator
- Enter
10.20.30.45in the IPv4 address field - Enter
20as the CIDR prefix length - Read the live result, or choose “Load example” to fill both fields automatically
The expected result is:
- Subnet mask:
255.255.240.0 - Network address:
10.20.16.0 - Broadcast address:
10.20.31.255 - Usable host range:
10.20.16.1 – 10.20.31.254 - Total addresses:
4096 - Usable hosts:
4094
Why does the third octet change from 30 to 16? After the first two octets, /20 uses the first four bits of the third octet for the network. That creates blocks of 16, and 30 falls inside the 16–31 block.
Why /31 and /32 Need Special Handling
“Total addresses minus two” is a useful shortcut for ordinary broadcast subnets, but it does not describe /31 and /32 correctly.
/31: Two Endpoints on a Point-to-Point Link
A /31 contains exactly two addresses. Under the point-to-point usage described in RFC 3021, both addresses can serve as link endpoints, so the calculator shows both as usable. For example, the range for 192.0.2.10/31 is 192.0.2.10 – 192.0.2.11.
/32: One IPv4 Address
A /32 has no host bits and identifies one address. For 192.0.2.10/32, the network address, broadcast address, and host range all resolve to 192.0.2.10; total and usable address counts are both 1.
Practical Ways to Use Subnet Results
Check Whether Two IP Addresses Share a Subnet
Calculate both IP addresses with the same prefix. If their network addresses match, they are in the same subnet. If the network addresses differ, communication normally requires routing.
Validate a DHCP Pool
Calculate the usable host range, then verify that both endpoints of the DHCP pool fall within it. In an ordinary subnet, do not assign the network or broadcast address to a client.
Review an ACL or Firewall Range
ACLs often use CIDR notation or wildcard masks. Generate both representations and compare them with the device configuration line by line. The text replacement tool can help normalize repeated fields in a configuration snippet, but review the result before applying it to a device.
FAQ
Why Is the Network Address Different from 192.168.1.10 for /24?
The input is a host address inside the subnet. Applying the /24 mask clears its host bits, producing the network address 192.168.1.0.
Does a Larger Prefix Mean a Larger Subnet?
No. A larger prefix reserves more bits for the network and leaves fewer for hosts. A /28 has 16 total addresses, while a /24 has 256.
Does the Tool Upload My IP Address?
No. The page performs the calculation in the current browser, and this tool does not upload the IP address you enter.
Can It Calculate IPv6 Subnets?
No. This page validates four-part IPv4 addresses and accepts CIDR prefixes from 0 to 32; it does not implement IPv6 subnet calculation.
Summary
For a reliable IPv4 subnet calculation, verify the IP address and CIDR prefix, then check the network address, broadcast address, and usable range. Treat /31 and /32 according to their special rules. Before applying any result to production networking, compare it with the actual address plan and device requirements.