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IPv4 subnet calculator

Enter a host IP, original mask and new mask to get subnet bits, hosts, network, first and last host, broadcast, and the full subnet table.

You can also paste the address with its prefix, such as 192.168.10.75/24, and the original mask is set for you.

The tool counts all 2^n subnets and numbers them from SID 0, because subnet zero and the all-ones subnet are allowed on modern networks and in current Cisco courses. Usable hosts are 2^h − 2, leaving out the network and broadcast addresses, except /31 point-to-point links (RFC 3021) and /32 single hosts. Large tables show 256 rows per page, and the CSV holds up to 65,536 subnets. Nothing you type is stored or sent.

Subnet a network from a host IP and two masks

Enter the host IP address, the original subnet mask, and the new subnet mask. The tool shows the number of subnet bits (new prefix minus original prefix), the number of subnets (2 to that power), the host bits (32 minus the new prefix), and the usable hosts per subnet (2 to the host bits, minus 2). It then finds the subnet that holds your host, with its network address, first host, last host, and broadcast address, and lists every subnet in a table you can download as CSV.

Worked example: 192.168.10.75, /24 to /27

27 minus 24 is 3 subnet bits, so there are 8 subnets. 32 minus 27 leaves 5 host bits, so each subnet has 30 usable hosts. The block size is 256 minus 224, or 32, so the subnets start at .0, .32, .64, and so on. The host .75 sits in SID 2: network 192.168.10.64, first host .65, last host .94, broadcast .95. Use it to check your hand computations for CCNA, TESDA, or your ETEEAP Computer Engineering subjects, and plan study time with the study schedule planner.

Questions

Why subtract 2 from the hosts per subnet?
The first address of each subnet is the network address and the last is the broadcast address. Neither can be given to a device.
Does SID 0 count as a subnet?
Yes. Modern networks and current Cisco courses use subnet zero and the all-ones subnet, so all 2^n subnets are counted. If an older book uses 2^n − 2, leave out the first and last rows.
What about /31 and /32?
A /31 has two addresses for a point-to-point link, both usable, with no broadcast (RFC 3021). A /32 is a single host.
Can the new mask be shorter than the original?
No. Subnetting borrows bits from the host part, so the new prefix must be equal or longer. A shorter mask is supernetting, which is a different calculation.
Is anything I type sent or saved?
No. Everything is calculated in your browser.