Subnet / CIDR Calculator

Frequently Asked Questions

What is CIDR notation?

CIDR notation writes a network as an IP followed by a slash and prefix length, like 192.168.1.0/24. The prefix length counts network bits and replaced the old Class A/B/C system.

Why do I lose 2 addresses in every subnet?

The first address (all host bits zero) is the network address used in routing. The last (all host bits one) is the broadcast address. Neither can be assigned to a device.

What are the RFC 1918 private address ranges?

10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 are reserved for private networks and never routed on the public internet.

What is a wildcard mask and how does it differ from a subnet mask?

A wildcard mask is the bitwise inverse of the subnet mask. Cisco ACLs and OSPF use wildcards instead of subnet masks. The Wildcard Mask Calculator covers non-contiguous wildcards for advanced ACL patterns.

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How It Works

This tool takes an IPv4 address and a CIDR prefix and performs the bitwise math that defines a subnet. The prefix length builds a 32-bit subnet mask (a /24 means 24 one-bits followed by 8 zero-bits = 255.255.255.0). Bitwise-ANDing the address with the mask yields the network address; ORing the network with the inverted mask gives the broadcast address.

Total addresses equal 2^(32 - prefix). Usable hosts are that minus 2 (one for the network address, one for broadcast) - except /31 (2 usable, RFC 3021 point-to-point) and /32 (1 address, a single host route). The wildcard mask (the bitwise inverse of the subnet mask) is shown for use in Cisco ACLs and OSPF network statements.

Use Cases

Network engineers and administrators use this calculator daily to:

Determine the network address, broadcast, and usable host range for any given IP and prefix.

Verify that two IP addresses are on the same subnet before troubleshooting a connectivity problem.

Generate the correct wildcard mask for a Cisco ACL or OSPF network statement directly from the subnet mask.

Plan IP address allocation for a new VLAN or network segment.

Confirm DHCP scope boundaries before configuring a DHCP server.

Tips

Remember the -2 rule for usable hosts - every /28 or larger subnet loses the network address (all zeros) and broadcast address (all ones) as unusable. A /24 serves 254 devices, not 256.

Use RFC 1918 private ranges for all internal networks: 10.0.0.0/8 (16+ million addresses), 172.16.0.0/12 (1 million), 192.168.0.0/16 (65,534). This prevents address collisions with public internet space.

Size point-to-point links correctly - a WAN link between two routers needs only a /30 (2 usable hosts) or /31 (RFC 3021), not a /24 that wastes 252 addresses.

Overlapping subnets cause routing failures - if two VPNs or sites both use 192.168.1.0/24, routing becomes ambiguous. Plan a unique addressing scheme across all connected sites.

Each step down the prefix doubles the address count - a /23 holds two /24s; a /16 holds 256 /24s. Use this relationship to quickly estimate how many devices a block supports.

FAQ

What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation writes a network as an IP address followed by a slash and a prefix length, like 192.168.1.0/24. The prefix length counts how many bits of the 32-bit address are fixed for the network. It replaced the old Class A/B/C system, which wasted huge blocks of addresses.

Why do I lose 2 addresses in every subnet?

The first address (all host bits zero) is the network address, used to identify the subnet in routing tables. The last address (all host bits one) is the broadcast address, which sends packets to all devices on the subnet simultaneously. Neither can be assigned to a device.

What are the RFC 1918 private address ranges?

10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 are reserved for private networks and are never routed on the public internet. Also note 169.254.0.0/16 (APIPA/link-local, assigned by the OS when DHCP fails) and 127.0.0.0/8 (loopback).

What is a wildcard mask and how does it differ from a subnet mask?

A wildcard mask is the bitwise inverse of the subnet mask. Where a subnet mask uses 1s to identify the network portion, a wildcard uses 0s for bits that must match and 1s for bits that are ignored. Cisco ACLs and OSPF use wildcards. The Wildcard Mask Calculator provides more detail on non-contiguous wildcards for advanced ACL patterns.

Subnet / CIDR Calculator

Frequently Asked Questions

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