The Comprehensive CIDR Subnetting Guide: Network Masks, Broadcast IPs & Usable Hosts

In the early days of the ARPANET and Internet, IPv4 addresses were divided into rigid classes: - **Class A (/8):** 16.7 million hosts per network (extremely wasteful) - **Class B (/16):** 65,534 hosts per network - **Class C (/24):** 254 hosts per network (often too small for growing organizations) To halt the rapid depletion of IPv4 addresses in 1993, the IETF introduced **Classless Inter-Domain Routing (CIDR)** via RFC 1519. CIDR allows subnet masks of arbitrary bit-lengths (such as `/27` or `/22`), empowering network engineers to allocate exact IP blocks for cloud VPCs (AWS, Google Cloud, Azure) and on-premise infrastructure.

Calculating IP boundaries requires understanding 32-bit binary arithmetic: - **Total IP Addresses:** $2^{(32 - \text{prefix})}$ - **Usable Host Addresses:** $2^{(32 - \text{prefix})} - 2$ (subtracting Network ID and Broadcast IP) - **Network ID:** Bitwise `AND` operation between the IP address and the Subnet Mask - **Broadcast IP:** Network ID bitwise `OR` inverted Subnet Mask (Wildcard Mask)

export function calculateSubnet(ipStr: string, prefix: number) {
  const ipParts = ipStr.split('.').map(Number);
  const ipInt = (ipParts[0] << 24) | (ipParts[1] << 16) | (ipParts[2] << 8) | ipParts[3];

  // Generate 32-bit subnet mask
  const maskInt = prefix === 0 ? 0 : (~0 << (32 - prefix));
  
  // Bitwise calculations
  const networkInt = (ipInt & maskInt) >>> 0;
  const broadcastInt = (networkInt | (~maskInt >>> 0)) >>> 0;
  const totalHosts = Math.pow(2, 32 - prefix);
  const usableHosts = prefix >= 31 ? 0 : totalHosts - 2;

  return { networkInt, broadcastInt, totalHosts, usableHosts };
}