IPv4 Subnet Mask Calculator
Enter an IPv4 address and either a prefix length (like 24) or a dotted subnet mask (like 255.255.255.0).
Tip: You can also enter values like /27 in the second field.
What does “calculate IP mask” mean?
When people search for “calculate IP mask,” they usually want to understand how an IP address is split into a network part and a host part. The subnet mask controls that split. In practical terms, it tells your router and devices which addresses are in the same local network and which must be reached through a gateway.
For IPv4, an address has 32 bits. A subnet mask is also 32 bits, where leading 1s mark network bits and trailing 0s mark host bits. Example: 255.255.255.0 is equivalent to /24, meaning the first 24 bits are network bits.
Why subnet masks matter
- Network design: Choose subnet sizes for offices, servers, IoT, or guest Wi-Fi.
- Routing: Routers compare destination IPs against masks to forward traffic correctly.
- Troubleshooting: Wrong masks cause “same LAN” devices to fail communication.
- Security segmentation: Smaller subnets make it easier to isolate systems.
How this calculator works
The calculator computes key subnet values from your inputs:
- Subnet mask and CIDR prefix
- Network address
- Broadcast address
- Wildcard mask
- Usable host range
- Total and usable host counts
- Address class and private/public indication
It supports both dotted masks and prefix notation, validates non-contiguous masks, and handles edge prefixes like /31 and /32.
Manual method to calculate a subnet mask
1) Convert to binary
Write both the IP address and subnet mask in binary. For example:
192.168.1.10 → 11000000.10101000.00000001.00001010
255.255.255.0 → 11111111.11111111.11111111.00000000
2) Find the network address
Perform bitwise AND between the IP and mask. Any host bits become 0.
Result: 192.168.1.0.
3) Find the broadcast address
Keep network bits unchanged, set all host bits to 1.
Result: 192.168.1.255.
4) Determine host range
For normal subnets, usable hosts are from network+1 to broadcast-1.
Example: 192.168.1.1 through 192.168.1.254.
2^(32 - prefix).
For /24, that is 2^8 = 256 addresses.
Common subnet sizes (IPv4)
- /24 → 256 total, 254 usable (typical small LAN)
- /27 → 32 total, 30 usable (small segment)
- /30 → 4 total, 2 usable (legacy point-to-point)
- /31 → 2 total, 2 usable in RFC 3021 point-to-point links
- /32 → single host route
CIDR vs dotted decimal mask
Both notations describe the same subnet boundary:
/8=255.0.0.0/16=255.255.0.0/24=255.255.255.0/26=255.255.255.192
CIDR is shorter and preferred in modern routing documentation, but dotted masks still appear often in device configuration screens.
Troubleshooting subnet mistakes
Devices cannot ping each other
Verify both IP and mask on each device. A mismatch like /24 vs /16 can make hosts disagree about whether a destination is local.
Intermittent connectivity
Check for overlapping subnets, duplicate static IP assignments, or an incorrect gateway outside the subnet range.
ACLs and wildcard masks
Remember wildcard mask is the inverse of subnet mask. For 255.255.255.0, wildcard is 0.0.0.255. Many firewall/router rule systems rely on this.
Final thoughts
A reliable IP mask calculator saves time and avoids costly addressing errors. Whether you are preparing for a networking exam, configuring a home lab, or managing enterprise VLANs, understanding subnet boundaries is a core skill. Use the calculator above for quick results, then verify your design with documentation and consistent IP planning.