code: func hostMask(ip net.IP) net.IPMask {
  	var hl int
  	if ip.To4() != nil {
  		hl = net.IPv4len * 8
  	} else {
  		hl = net.IPv6len * 8
  	}
  	return net.CIDRMask(hl, hl)
  }
docs: // hostMask returns an IP mask that corresponds with a host prefix.