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CIDR & subnet calculator

Enter an IPv4 CIDR block to get its network and broadcast addresses, netmask, usable host count and first/last usable IP. For smaller blocks it lists every usable address, so you can assign one IP per PowerMTA virtual MTA. Calculated in your browser.

CIDR block (IPv4)

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Subnet details

Usable IPs

CIDR calculator FAQ

What is this CIDR calculator for?+

It turns a CIDR block (like 203.0.113.0/24 or a /22) into the details you need to plan sending infrastructure: network and broadcast addresses, netmask, usable host count, and the first/last usable IP. For smaller blocks it lists every usable IP so you can assign one per PowerMTA virtual MTA.

How many usable IPs are in a /24 or a /22?+

A /24 has 256 total addresses and 254 usable (network and broadcast are reserved). A /22 has 1,024 total and 1,022 usable. In general, usable hosts = 2^(32 − prefix) − 2 for prefixes up to /30; a /31 has 2 (point-to-point) and a /32 is a single address.

Why does my host IP get changed to a network address?+

If you enter an address with host bits set (like 203.0.113.5/24), the calculator masks it to the network address (203.0.113.0) because that's what defines the block. The network and broadcast addresses are reserved and not usable for sending.

Does it support IPv6?+

This calculator focuses on IPv4, which is what email IP pools almost always use for reputation. IPv6 subnetting works differently and isn't covered here.