swapip.go

package main

import (
	"fmt"
	"net"
)

func main() {
	ip := net.ParseIP("1.2.3.4")
	mask := net.IPMask(net.ParseIP("255.255.0.0"))
	octetIP := net.ParseIP("123.123.123.123")
	swapped := combineMaskedIP(ip.Mask(inverseMask(mask)), octetIP.Mask(mask))

	fmt.Println(swapped) // Expect: 123.123.3.4
}

func inverseMask(a net.IPMask) net.IPMask {
	r := make(net.IPMask, len(a))
	for i := range a {
		r[i] = ^a[i]
	}
	return r
}

func combineMaskedIP(a, b net.IP) net.IP {
	if len(a) != len(b) {
		panic("addresses must be the same type")
	}
	r := make(net.IP, len(a))
	for i := range a {
		r[i] = a[i] | b[i]
	}
	return r
}