Tip: Enter only a prefix (like 24) if you only need subnet/wildcard masks. Add an IP to compute network and broadcast details.
What is a mask bit calculator?
A mask bit calculator helps you convert CIDR notation (like /24) into practical subnet information you can use immediately. Instead of memorizing every subnet mask, you enter the prefix length and instantly get:
- Subnet mask in dotted decimal (for example,
255.255.255.0) - Wildcard mask (useful in ACL and routing contexts)
- Total addresses and usable host count
- Network and broadcast addresses (when an IP address is provided)
How to use this calculator
1) Enter your mask bits
Type a whole number from 0 to 32. This is your CIDR prefix. A larger number means more network bits and fewer host bits.
2) Optionally enter an IPv4 address
If you supply an address like 10.20.30.40, the calculator can determine the exact network, broadcast, and host range for that subnet.
3) Click calculate
You will see the full subnet breakdown in one output block, including binary mask representation for quick verification.
Understanding the output fields
CIDR notation
This is your original prefix, shown in slash format such as /16 or /27.
Subnet mask
The dotted decimal version of the prefix. For instance, /26 becomes 255.255.255.192.
Wildcard mask
The inverse of the subnet mask. This is commonly used in Cisco ACLs and some routing protocol statements.
Total addresses vs usable hosts
Total addresses include network and broadcast in traditional IPv4 subnets. Usable host count usually subtracts two addresses, except special cases:
- /31: often used for point-to-point links, both addresses are usable
- /32: single host route, exactly one address
CIDR quick reference
- /8 → 255.0.0.0 → 16,777,216 total addresses
- /16 → 255.255.0.0 → 65,536 total addresses
- /24 → 255.255.255.0 → 256 total addresses
- /27 → 255.255.255.224 → 32 total addresses
- /30 → 255.255.255.252 → 4 total addresses
- /32 → 255.255.255.255 → 1 total address
How the math works behind the scenes
Subnet mask creation
An IPv4 address has 32 bits. A prefix of /N means the first N bits are set to 1 in the subnet mask, and the remaining bits are set to 0.
Network address
The network address is found using a bitwise AND between the IP address and subnet mask.
Broadcast address
The broadcast address sets all host bits to 1. It can be calculated as network OR wildcard mask.
Common mistakes this tool helps prevent
- Mixing up /26 and /27 host counts
- Using the wrong wildcard in firewall or ACL configurations
- Assigning broadcast or network addresses to end devices
- Incorrectly planning subnet ranges during VLAN segmentation
FAQ
Is this calculator for IPv4 only?
Yes, this page calculates IPv4 mask bits and subnet values.
Can I leave the IP field blank?
Absolutely. You can still convert CIDR to subnet/wildcard masks and host capacity without entering an IP.
Why does /31 show usable hosts?
Modern networking standards allow /31 for point-to-point links where no broadcast is needed, so both addresses are considered usable.
Final note
A fast mask bit calculator saves time, reduces subnetting mistakes, and makes network planning more reliable. Keep this page handy whenever you design LANs, configure routers, or validate IP plans.