go-network[2]-ipmask

/* Mask
 */

package main

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

func main() {
	if len(os.Args) != 2 {
		fmt.Fprintf(os.Stderr, "Usage: %s dotted-ip-addr\n", os.Args[0])
		os.Exit(1)
	}
	dotAddr := os.Args[1]

	addr := net.ParseIP(dotAddr)
	if addr == nil {
		fmt.Println("Invalid address")
		os.Exit(1)
	}
	mask := addr.DefaultMask()
	network := addr.Mask(mask)
	ones, bits := mask.Size()
	fmt.Println("Address is ", addr.String(),
		" Default mask length is ", bits,
		"Leading ones count is ", ones,
		"Mask is (hex) ", mask.String(),
		" Network is ", network.String())
	os.Exit(0)
}

$ go build .
$ ./net 127.0.0.1
Address is  127.0.0.1  Default mask length is  32 Leading ones count is  8 Mask is (hex)  ff000000  Network is  127.0.0.0