func ipv4only(addr IPAddr) bool {
	return addr.IP.To4() != nil
}

// ipv6only reports whether addr is an IPv6 address except IPv4-mapped IPv6 address.
func ipv6only(addr IPAddr) bool {
	return len(addr.IP) == IPv6len && addr.IP.To4() == nil
}

// SplitHostPort splits a network address of the form "host:port",
// "host%zone:port", "[host]:port" or "[host%zone]:port" into host or
// host%zone and port.
//