What is the difference between CIDR and subnet mask?

Both describe the same thing - where the network part of an address ends and the host part begins. CIDR notation uses a slash followed by the prefix length (e.g. /24), while a subnet mask writes it as a dotted-decimal number (e.g. 255.255.255.0). CIDR is more compact and is the modern standard in routing and network documentation.

Why are network and broadcast addresses reserved?

The first address in a subnet (e.g. 192.168.1.0/24) identifies the network itself, and the last address (192.168.1.255) is the broadcast address used to send packets to all hosts in the subnet. Neither can be assigned to individual devices, which is why a /24 subnet has 254 usable host addresses, not 256.

How do I split a network into smaller subnets?

Increase the prefix length to divide the network. For example, a /24 (256 addresses) can be split into two /25 subnets (128 addresses each) or four /26 subnets (64 each). Each step adds one bit to the prefix, halving the number of available addresses per subnet.

What is a wildcard mask?

A wildcard mask is the bitwise inverse of a subnet mask, used in Cisco ACLs and OSPF configuration. For subnet mask 255.255.255.0, the wildcard is 0.0.0.255. It tells the router which bits to match (0) and which to ignore (1).

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IPv4 calculator

Calculate subnets, CIDR notation, netmask and address ranges for any IPv4 address. Enter an IP and prefix length to see network boundaries, broadcast address and the number of usable hosts.

IP address

Mask or prefix

Enter IPv4 and mask (dotted) or prefix (/24).

IP/Mask	—
IP/Prefix	—
Binary IP	—
Binary mask	—
Network	—
IPs available in network	—
Network address	—
Broadcast address	—

Reference: reserved blocks

127.0.0.0/8	Loopback, address of the current device.
10.0.0.0/8	Private (internal) networks
100.64.0.0/10	For use in service provider networks (CGNAT)
169.254.0.0/16	Link-local addresses are often used for automatic network address configuration when external sources are unavailable.
172.16.0.0/12	Private (internal) networks
192.0.0.0/29	Dual-Stack Lite
192.0.2.0/24	For documentation examples
192.168.0.0/16	Private (internal) networks
198.51.100.0/24	For documentation examples
203.0.113.0/24	For documentation examples
224.0.0.0/4	For multicast
240.0.0.0/4	Reserved for future use
255.255.255.255/32	Broadcast address

What it is

The IPv4 calculator derives network, host range, mask and broadcast from an address and CIDR prefix.

How it works

Enter an IPv4 like 192.168.1.10/24 or address + mask. The tool shows network, min/max host and address count.

Examples

10.0.0.1/8 → network 10.0.0.0, hosts 10.0.0.1–10.255.255.254.
192.168.1.0/26 → 62 hosts.

Limitations & notes

Calculations follow classic IPv4 rules. For VLSM, validate each subnet separately. Private ranges are not routed on the public Internet.

FAQ

What is the difference between CIDR and subnet mask? Both describe the same thing - where the network part of an address ends and the host part begins. CIDR notation uses a slash followed by the prefix length (e.g. /24), while a subnet mask writes it as a dotted-decimal number (e.g. 255.255.255.0). CIDR is more compact and is the modern standard in routing and network documentation.
Why are network and broadcast addresses reserved? The first address in a subnet (e.g. 192.168.1.0/24) identifies the network itself, and the last address (192.168.1.255) is the broadcast address used to send packets to all hosts in the subnet. Neither can be assigned to individual devices, which is why a /24 subnet has 254 usable host addresses, not 256.
How do I split a network into smaller subnets? Increase the prefix length to divide the network. For example, a /24 (256 addresses) can be split into two /25 subnets (128 addresses each) or four /26 subnets (64 each). Each step adds one bit to the prefix, halving the number of available addresses per subnet.
What is a wildcard mask? A wildcard mask is the bitwise inverse of a subnet mask, used in Cisco ACLs and OSPF configuration. For subnet mask 255.255.255.0, the wildcard is 0.0.0.255. It tells the router which bits to match (0) and which to ignore (1).

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