IP Subnet Calculator
Adjust the inputs below. Results update as you type.
How it works
A /24 subnet has 256 addresses (254 usable: subtract network and broadcast). Network address = IP AND subnet mask. Broadcast = network address OR (inverted mask). CIDR notation /n means n bits set to 1 in the mask. Subnetting divides large networks for routing efficiency and security isolation. IPv4 exhaustion has driven migration to IPv6 with 128-bit addresses.
Input guidance
- Confirm date/time/unit settings before comparing outputs.
- If a task has optional fields, run both with and without them to understand impact.
- Save scenario variants when making practical decisions.
The formula
A subnet mask splits an IP address into network and host parts. The prefix /n sets n network bits; usable hosts = 2^(32−n) − 2 for IPv4.
Worked example
A /24 network (255.255.255.0) provides 2⁸ − 2 = 254 usable host addresses, reserving one for the network and one for broadcast.
More examples to test
- Quick-pass example: run default assumptions for a first estimate.
- Refined example: update one assumption at a time to isolate impact.
How to interpret results
Use outputs as operational estimates and pair them with local constraints, business rules, or provider requirements.
When this can be inaccurate
Utilities can miss local policy details, special-case rules, and environment-specific constraints.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
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General recommendation, not professional advice.
Frequently asked questions
What does the /24 notation mean?+
CIDR notation: the number of bits used for the network portion. /24 means 24 network bits, leaving 8 bits (256 addresses) for hosts.
Why subtract 2 from host count?+
The first address identifies the network and the last is the broadcast address, so neither can be assigned to a device.
What is a subnet mask for?+
It tells devices which part of an IP is the network and which is the host, controlling how traffic is routed locally versus externally.
Why are outputs slightly different from another tool?+
Different tools can use different rounding rules, default assumptions, and treatment of edge cases.
How can I improve estimate reliability?+
Use verified inputs from real records and re-run calculations after major assumption changes.