How does a prefix length map to a subnet mask?

The prefix is the number of leading 1 bits. /24 sets the first 24 bits to 1, giving 11111111.11111111.11111111.00000000 = 255.255.255.0.

What is a wildcard mask?

It is the bitwise inverse of the subnet mask (255.255.255.0 becomes 0.0.0.255). Cisco access lists and OSPF use wildcard masks to match address ranges.

How is the usable host count calculated?

For a prefix p, the block holds 2^(32 − p) addresses, and two are reserved (network and broadcast), so usable hosts = 2^(32 − p) − 2. A /24 gives 2^8 − 2 = 254 usable hosts.

Why do /31 and /32 show no usable hosts here?

The standard formula subtracts the network and broadcast addresses, which leaves zero for a /31 (2 addresses) and /32 (1 address). These prefixes are used for point-to-point links and single hosts via special rules, but the classic usable-host count is zero.

Why does my mask show an error?

A valid IPv4 subnet mask must have all its 1 bits contiguous from the left. Values like 255.0.255.0 are rejected because the 1 bits are not contiguous.

Is anything uploaded?

No. The conversion runs entirely in your browser; nothing you type is sent to a server.

Subnet Mask Converter

Switch in either direction between a CIDR prefix (/24) and a dotted-decimal subnet mask (255.255.255.0). The tool also derives the wildcard mask and the number of usable hosts, making it a quick reference when configuring routers, firewalls, and IP plans.

How it works

A subnet mask is just a run of leading 1 bits followed by 0 bits:

Prefix to mask: set the first p bits to 1. For /24 that is 11111111.11111111.11111111.00000000 = 255.255.255.0.
Mask to prefix: count the leading 1 bits — but only if they are contiguous. A mask like 255.0.255.0 is rejected because its 1 bits are not all on the left.
Wildcard mask = bitwise NOT of the subnet mask, e.g. 0.0.0.255 for a /24.
Usable hosts = 2^(32 − p) − 2, subtracting the network and broadcast addresses.

Example

Converting /26:

Mask = 255.255.255.192
Wildcard = 0.0.0.63
Usable hosts = 2^(32 − 26) − 2 = 2^6 − 2 = 62

Prefix	Subnet mask	Wildcard	Usable hosts
/24	255.255.255.0	0.0.0.255	254
/26	255.255.255.192	0.0.0.63	62
/30	255.255.255.252	0.0.0.3	2

Everything runs in your browser; nothing is uploaded.