Subnet Mask Calculator

Frequently Asked Questions

What is the difference between a subnet mask and a CIDR prefix?

They express the same thing in different formats. /24 is a count of network bits; 255.255.255.0 is the same information as a 32-bit dotted-decimal number. CIDR is more compact and modern.

What is a valid subnet mask?

A valid subnet mask is a contiguous block of 1s followed by 0s: e.g., 255.255.255.0 (/24). A non-contiguous value like 255.0.255.0 is invalid as a subnet mask.

Why does /31 have 2 usable hosts but /30 also has only 2?

/30 has 4 total addresses (2 usable + network + broadcast). /31 has 2 total and per RFC 3021 both are usable since no network/broadcast is reserved on point-to-point links.

Can I use 0.0.0.0/0 in this calculator?

Yes. /0 has 4,294,967,296 total addresses. It represents the default route matching any IPv4 address, used in routing tables to represent the entire internet.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How It Works

This tool works in two modes. In CIDR mode you enter a prefix length (0-32) and it returns the dotted-decimal subnet mask, total address count, and usable host count. In IP Range mode you supply a network IP and dotted-decimal mask, and it computes the network address, broadcast address, first and last usable hosts, and the equivalent CIDR prefix.

A subnet mask is a 32-bit number whose 1 bits mark the network portion and 0 bits mark the host portion. For a prefix length n: Total = 2^(32 - n) and Usable = Total - 2. The network address is IP AND mask (bitwise AND). The broadcast is the network OR the inverted mask. Special cases: /31 has 2 usable addresses per RFC 3021 (point-to-point); /32 has exactly 1 address (a host route).

Use Cases

Network engineers and help desk staff use this calculator to:

Quickly convert a CIDR prefix like /24 to its dotted-decimal mask 255.255.255.0 for use in device configuration interfaces that require the mask format.

Verify a network configuration by entering the device's IP and mask and confirming the calculated network and broadcast addresses match expectations.

Determine the usable host count for a given mask when sizing subnets for a new deployment.

Confirm DHCP pool boundaries match the subnet's first and last usable addresses.

Translate between the two common notations (CIDR and dotted mask) when reading documentation written in one format and configuring a system expecting the other.

Tips

Memorize the common masks: /24 = 255.255.255.0 (254 hosts), /25 = 255.255.255.128 (126 hosts), /26 = 255.255.255.192 (62 hosts), /27 = 255.255.255.224 (30 hosts), /28 = 255.255.255.240 (14 hosts), /30 = 255.255.255.252 (2 hosts).

Each bit added to the prefix halves the address count - /23 has 512 addresses; /24 has 256; /25 has 128. Use this doubling rule to quickly estimate block sizes.

The host bits are the subnet ID in VLSM - the bits between the prefix and the full 32 bits identify individual subnets when you sub-divide a block; see the VLSM Subnet Planner for details.

CIDR notation is always preferred in documentation - 10.0.0.0/8 is unambiguous; 10.0.0.0 with mask 255.0.0.0 requires two fields and is more error-prone in writing.

The /31 exception - RFC 3021 allows /31 for point-to-point links, treating both addresses as usable (no network/broadcast reservation). Most modern router OSes support this; verify your device does before using it.

FAQ

What is the difference between a subnet mask and a CIDR prefix?

They express the same thing in different formats. A CIDR prefix like /24 is a single number counting the network bits. A subnet mask like 255.255.255.0 is a 32-bit dotted-decimal representation of the same information: 24 one-bits followed by 8 zero-bits. The CIDR format is more compact and modern; subnet masks are still required by some older device interfaces.

What is a valid subnet mask?

A valid subnet mask must be a contiguous sequence of 1s followed by a contiguous sequence of 0s - for example, 255.255.255.0 (/24) or 255.255.248.0 (/21). A value like 255.0.255.0 is not a valid subnet mask (non-contiguous) and cannot be expressed as a CIDR prefix. Wildcard masks are non-contiguous by design but are a different construct.

Why does /31 have 2 usable hosts but /30 has only 2?

/30 has 4 total addresses: the network address, 2 usable hosts, and the broadcast address - that is 2 usable. /31 has 2 total addresses and per RFC 3021 both are usable as the network and broadcast distinction is dropped for point-to-point links - also 2 usable. The difference is /30 wastes 2 addresses on unusable network/broadcast while /31 does not.

Can I use 0.0.0.0/0 in this calculator?

Yes. A /0 prefix means all 32 bits are in the host portion: 1 network containing 2^32 = 4,294,967,296 addresses with 4,294,967,294 usable. This is the default route that matches any IPv4 address, used in routing tables to represent the entire internet.

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