Subnet & CIDR Calculator

Local processing guarantee: IP math executed entirely client-side. No data is sent to servers.

Network Configuration

IP Address

Subnet Mask / CIDREnter CIDR range (e.g. 24) or Dotted-decimal (e.g. 255.255.255.0).

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Understanding IPv4 Subnetting & CIDR

Subnetting is the practice of dividing a physical or logical network into smaller, logical sub-networks (subnets). CIDR (Classless Inter-Domain Routing) was introduced in 1993 to replace the older, rigid Classful network design (Class A, B, and C). It uses the variable-length subnet masking (VLSM) notation where the slash (/) signifies the exact number of bits reserved for the Network ID.

RFC 1918 Private Ranges

Private IP addresses are reserved for internal use within a local area network (LAN) and are not routable on the public internet. They include:

10.0.0.0/8 (10.0.0.0 – 10.255.255.255): Single large network.
172.16.0.0/12 (172.16.0.0 – 172.31.255.255): 16 contiguous Class B networks.
192.168.0.0/16 (192.168.0.0 – 192.168.255.255): 256 local networks, extremely common for home networks.

Special Addresses

Within any IPv4 subnet, the first and last address are reserved for special purposes:

Network Address: The first address in the subnet (host bits all set to 0). Identifies the subnet itself.
Broadcast Address: The last address in the subnet (host bits all set to 1). Used to transmit packets to all hosts in the subnet.
Gateway: Typically the first usable address (e.g. .1) or last usable address (e.g. .254) is assigned to the local router interface.

Frequently Asked Questions

What is a subnet?

A subnet (subnetwork) is a logical segment of a larger IP network. Subnetting splits a network into smaller isolated segments, improving security, reducing broadcast traffic, and allowing efficient IP address allocation.

What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation writes a network as IP/prefix — e.g. 192.168.1.0/24. The /24 means 24 bits are the network portion, leaving 8 bits (256 addresses) for hosts in that subnet.

What's the difference between network address and broadcast address?

The network address (first IP, e.g. 192.168.1.0) identifies the subnet and cannot be assigned to a host. The broadcast address (last IP, e.g. 192.168.1.255) sends to all hosts. Usable host addresses are everything in between.

How many usable hosts does a /24 subnet have?

A /24 has 256 addresses (2^8). Subtract 1 for the network address and 1 for the broadcast, giving 254 usable host IPs. General formula: 2^(32-prefix) - 2. For a /25: 128 - 2 = 126 usable hosts.

What's the difference between a private and public IP address?

Private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are not routable on the public internet — used for internal networks. Public IPs are globally routable. Your router uses NAT to translate between them.

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