IP Subnet Calculator

The IP Subnet Calculator performs subnet calculations for the given network address, subnet mask, and IP address. It supports both IPv4 and IPv6 subnetting with complete CIDR conversion, usable host IP ranges, broadcast addresses, wildcard masks, and binary representations.

IPv4 Subnet Calculator

IPv6 Subnet Calculator

Typical Subnets for IPv4:

Prefix Subnet Mask Usable Hosts per Subnet Total Subnets
Class A
/8 255.0.0.0 16,777,214 1
/9 255.128.0.0 8,388,606 2
/10 255.192.0.0 4,194,302 4
/11 255.224.0.0 2,097,150 8
/12 255.240.0.0 1,048,574 16
/13 255.248.0.0 524,286 32
/14 255.252.0.0 262,142 64
/15 255.254.0.0 131,070 128
Class B
/16 255.255.0.0 65,534 1
/17 255.255.128.0 32,766 2
/18 255.255.192.0 16,382 4
/19 255.255.224.0 8,190 8
/20 255.255.240.0 4,094 16
/21 255.255.248.0 2,046 32
/22 255.255.252.0 1,022 64
/23 255.255.254.0 510 128
Class C
/24 255.255.255.0 254 1
/25 255.255.255.128 126 2
/26 255.255.255.192 62 4
/27 255.255.255.224 30 8
/28 255.255.255.240 14 16
/29 255.255.255.248 6 32
/30 255.255.255.252 2 64
/31 255.255.255.254 0 (2 for RFC 3021) 128
/32 255.255.255.255 1 256
Subnet & Fraction Arithmetic Measurement Solver

Network bandwidth allocations, server rack unit clearances, and sub-block division ratios frequently require exact fraction arithmetic. Add, subtract, multiply, or divide mixed numbers and fractional values with step-by-step reduction and decimal conversion.

Common Presets:

The Complete IPv4 & IPv6 Subnetting & CIDR Architecture Guide

Subnetting is the fundamental mathematical technique used in computer networking to divide a large physical or virtual network into smaller, logically organized sub-networks (subnets). By partitioning IP address spaces, network architects enhance security through access-control isolation, contain broadcast storms, reduce latency, and ensure maximum conservation of IPv4 addresses. Understanding Classless Inter-Domain Routing (CIDR) and bitwise subnet masks is an indispensable prerequisite for network engineers, cloud infrastructure architects (AWS VPC, Azure VNet, Google Cloud VPC), and cybersecurity professionals.

1. Mathematical Foundations of IPv4 Subnetting

An IPv4 address consists of 32 binary bits arranged into four 8-bit bytes called octets, separated by periods (e.g., 192.168.1.1). Every IP address is fundamentally divided into two segments: the Network Prefix (which identifies the subnet) and the Host Identifier (which identifies the individual interface or host on that subnet).

  • Bitwise AND Operation: A router determines the network address by performing a logical bitwise AND between the 32-bit IP address and the 32-bit subnet mask. Wherever the mask bit is 1, the IP bit is preserved; wherever the mask bit is 0, the result is forced to 0.
  • Total Host Formula: If a subnet mask reserves h host bits (where h = 32 - CIDR), the total number of IP addresses in the subnet is 2^h.
  • Usable Host Formula: In standard IPv4 networking, two addresses are reserved and cannot be assigned to endpoints: the Network Address (where all host bits are binary 0) and the Broadcast Address (where all host bits are binary 1). Therefore, the number of usable hosts is 2^h - 2.
  • Point-to-Point /31 Subnets (RFC 3021): For direct router-to-router point-to-point serial links, RFC 3021 eliminates dedicated network and broadcast addresses, allowing both addresses in a /31 block to be assigned to the two peer router interfaces, conserving vast blocks of IPv4 space.

2. Private IP Spaces (RFC 1918) vs. Public Routing

To prevent premature exhaustion of the global 4.29 billion IPv4 address pool, the Internet Engineering Task Force (IETF) designated three private IPv4 blocks under RFC 1918 that are guaranteed never to be routed on the public internet:

  • 10.0.0.0/8 (Class A Private): Range: 10.0.0.0 to 10.255.255.255 (16,777,216 total addresses). Ideal for large enterprise campus networks, global WANs, and cloud multi-tier VPC topologies.
  • 172.16.0.0/12 (Class B Private): Range: 172.16.0.0 to 172.31.255.255 (1,048,576 addresses across 16 contiguous /16 blocks). Frequently used in mid-sized enterprise networks, virtualization clusters, and Docker/Kubernetes container overlays.
  • 192.168.0.0/16 (Class C Private): Range: 192.168.0.0 to 192.168.255.255 (65,536 addresses across 256 contiguous /24 blocks). Standard default for home routers, small-office/home-office (SOHO) Wi-Fi networks, and lab benches.
  • 100.64.0.0/10 (Carrier-Grade NAT / CGNAT - RFC 6598): Range: 100.64.0.0 to 100.127.255.255 (4,194,304 addresses). Used by ISPs to aggregate multiple residential subscribers behind shared public IPv4 gateways.

3. IPv6 Architecture & The Mandatory /64 Prefix

IPv6 replaces IPv4's 32-bit limitation with a massive 128-bit address space, representing 2^128 (approximately 340 undecillion) unique addresses. IPv6 addresses are written as eight groups of four hexadecimal digits (hextets) separated by colons.

  • Global Routing Prefix (Typically /48 to /56): Assigned by Regional Internet Registries (RIRs) and ISPs to enterprises and organizations.
  • Subnet Identifier (Bits 48 through 64): Gives an enterprise up to 65,536 distinct /64 subnets from a single /48 assignment.
  • Interface Identifier (Bits 64 through 128): RFC 4291 mandates that standard subnets use a /64 prefix to allow Stateless Address Autoconfiguration (SLAAC). Each individual /64 subnet contains 18,446,744,073,709,551,616 unique host addresses.

4. Cloud VPC Subnetting (AWS, Azure & GCP)

When architecting subnets in major cloud environments such as Amazon Web Services (AWS VPC), Microsoft Azure Virtual Networks, and Google Cloud Platform (GCP VPC), cloud providers reserve additional IP addresses within each subnet for hypervisor infrastructure services:

  • .0: Network address.
  • .1: VPC router / default gateway address.
  • .2: DNS resolver service (Amazon Provided DNS or Azure DNS).
  • .3: Cloud internal management or broadcast emulation.
  • .255: Network broadcast address.

Consequently, in an AWS VPC, a /24 subnet provides 251 usable host IP addresses rather than the traditional on-premise count of 254. Keep this cloud reserve in mind when sizing subnets for Kubernetes node pools or elastic load balancer clusters.

Frequently Asked Questions (FAQ)

How do I find the network and broadcast address from an IP and CIDR? ▾
Convert both the IP address and the subnet mask into 32-bit binary integers. Perform a bitwise AND between the IP and mask to get the Network Address. Then, invert the subnet mask to calculate the Wildcard Mask, and perform a bitwise OR between the Network Address and Wildcard Mask to obtain the Broadcast Address.
Why does a /30 subnet only provide 2 usable IP addresses? ▾
A /30 prefix leaves 32 - 30 = 2 host bits. The total number of addresses is 2^2 = 4. Since the first address is reserved as the network identifier and the fourth address is reserved as the broadcast address, exactly 4 - 2 = 2 usable host addresses remain, making /30 the traditional standard for point-to-point WAN links.
What is the difference between classful and classless (CIDR) addressing? ▾
Classful addressing (invented in 1981 under RFC 791) rigidly categorized IP addresses into Class A (/8), Class B (/16), and Class C (/24) based on their leading octets, creating immense address wastage. Classless Inter-Domain Routing (CIDR, introduced in 1993 under RFC 1519) removed these fixed boundaries, allowing subnet masks of arbitrary prefix length (/1 through /32) and enabling flexible route aggregation.
How do wildcard masks work in Cisco router configurations? ▾
A wildcard mask tells the router which bits in an IP address must match (where wildcard bit is 0) and which bits can be ignored or vary (where wildcard bit is 1). It is calculated as 255.255.255.255 minus the subnet mask. For example, for a /26 subnet (mask 255.255.255.192), the wildcard mask is 0.0.0.63.