# Go: how to calculate the number of available IP addresses in a subnet

In this article, we will see how to implement a function in Go to calculate the number of available IP addresses in a given subnet.

Calculating the number of available IP addresses in a subnet is a crucial operation in network management, especially when predicting the size of a network or configuring networking for applications and services. Using the Go programming language, we can develop a utility that allows us to perform this calculation efficiently. In this article, we will see how to implement a function in Go to calculate the number of available IP addresses in a given subnet.

Before diving into the code, it is essential to understand some basic networking concepts:

• IP Address: A unique address assigned to each device on a network.
• Subnet Mask: Used to divide the address IP in a 'network' and a 'host part'.
• CIDR (Classless Inter-Domain Routing): A notation used to specify the subnet mask, e.g. `192.168.1.0/24` where `/24` indicates that the first 24 bits of the address are the network part.

The formula to calculate the number of available addresses in a subnet is:

For a subnet with a 24-bit netmask (commonly expressed as /24 or 255.255.255.0), the calculation would be:

2(32-24) = 256

Of these 256 addresses, one is reserved for the network address and one for the broadcast, thus leaving 254 usable addresses for hosts.

Here's how you might implement a function in Go that calculates the number of usable IP addresses in a subnet:

``````
package main

import (
"fmt"
"math"
"net"
"os"
)

// calculateIPs calculates the number of IP addresses that can be used in a CIDR subnet.
func calculateIPs(cidr string) (int, error) {
_, ipv4Net, err := net.ParseCIDR(cidr)
if err != nil {
return 0, err
}

totalIPs := int(math.Pow(2, float64(32-maskSize))) - 2
if totalIPs < 0 {
totalIPs = 0 // Handles the case of subnets with masks /31 and /32
}
}

func main() {
if len(os.Args) < 2 {
fmt.Println("Usage: go run main.go <CIDR>")
os.Exit(1)
}
cidr := os.Args[1]

count, err := calculateIPs(cidr)
if err != nil {