}

		if cfg.NodeCIDRMaskSizeIPv4 != 0 {
			ipv4Mask = int(cfg.NodeCIDRMaskSizeIPv4)
		}
		if cfg.NodeCIDRMaskSizeIPv6 != 0 {
			ipv6Mask = int(cfg.NodeCIDRMaskSizeIPv6)
		}
		return sortedSizes(ipv4Mask, ipv6Mask), nil
	}

	maskConfigured := cfg.NodeCIDRMaskSize != 0
	maskV4Configured := cfg.NodeCIDRMaskSizeIPv4 != 0

	{"0.0.0.0/24", IPv4(0, 0, 0, 0), &IPNet{IP: IPv4(0, 0, 0, 0), Mask: IPv4Mask(255, 255, 255, 0)}, nil},
	{"135.104.0.0/24", IPv4(135, 104, 0, 0), &IPNet{IP: IPv4(135, 104, 0, 0), Mask: IPv4Mask(255, 255, 255, 0)}, nil},
	{"135.104.0.1/32", IPv4(135, 104, 0, 1), &IPNet{IP: IPv4(135, 104, 0, 1), Mask: IPv4Mask(255, 255, 255, 255)}, nil},

	// true
	// false
	// true
	// false
}

func ExampleIP_Mask() {
	ipv4Addr := net.ParseIP("192.0.2.1")
	// This mask corresponds to a /24 subnet for IPv4.
	ipv4Mask := net.CIDRMask(24, 32)
	fmt.Println(ipv4Addr.Mask(ipv4Mask))

	ipv6Addr := net.ParseIP("2001:db8:a0b:12f0::1")
	// This mask corresponds to a /32 subnet for IPv6.
	ipv6Mask := net.CIDRMask(32, 128)
	fmt.Println(ipv6Addr.Mask(ipv6Mask))

	}
	if len(ip) == IPv6len {
		return ip
	}
	return nil
}

// Default route masks for IPv4.
var (
	classAMask = IPv4Mask(0xff, 0, 0, 0)
	classBMask = IPv4Mask(0xff, 0xff, 0, 0)
	classCMask = IPv4Mask(0xff, 0xff, 0xff, 0)
)

// DefaultMask returns the default IP mask for the IP address ip.
// Only IPv4 addresses have default masks; DefaultMask returns

		case *route.InterfaceAddrMessage:
			if index != 0 && index != m.Index {
				continue
			}
			var mask IPMask
			switch sa := m.Addrs[syscall.RTAX_NETMASK].(type) {
			case *route.Inet4Addr:
				mask = IPv4Mask(sa.IP[0], sa.IP[1], sa.IP[2], sa.IP[3])
			case *route.Inet6Addr:
				mask = make(IPMask, IPv6len)
				copy(mask, sa.IP[:])
			}
			var ip IP
			switch sa := m.Addrs[syscall.RTAX_IFA].(type) {

						ipsa := (*syscall.RawSockaddrInet4)(unsafe.Pointer(iprsa))
						nmsa := (*syscall.RawSockaddrInet4)(unsafe.Pointer(nmrsa))
						ip = IPv4(ipsa.Addr[0], ipsa.Addr[1], ipsa.Addr[2], ipsa.Addr[3])
						mask = IPv4Mask(nmsa.Addr[0], nmsa.Addr[1], nmsa.Addr[2], nmsa.Addr[3])
					case syscall.AF_INET6:
						ipsa := (*syscall.RawSockaddrInet6)(unsafe.Pointer(iprsa))
						nmsa := (*syscall.RawSockaddrInet6)(unsafe.Pointer(nmrsa))

		}

		if cfg.NodeCIDRMaskSizeIPv4 != 0 {
			ipv4Mask = int(cfg.NodeCIDRMaskSizeIPv4)
		}
		if cfg.NodeCIDRMaskSizeIPv6 != 0 {
			ipv6Mask = int(cfg.NodeCIDRMaskSizeIPv6)
		}
		return sortedSizes(ipv4Mask, ipv6Mask), nil
	}

	maskConfigured := cfg.NodeCIDRMaskSize != 0
	maskV4Configured := cfg.NodeCIDRMaskSizeIPv4 != 0

		{"IPAddr.Zone", Field, 1},
		{"IPConn", Type, 0},
		{"IPMask", Type, 0},
		{"IPNet", Type, 0},
		{"IPNet.IP", Field, 0},
		{"IPNet.Mask", Field, 0},
		{"IPv4", Func, 0},
		{"IPv4Mask", Func, 0},
		{"IPv4allrouter", Var, 0},
		{"IPv4allsys", Var, 0},
		{"IPv4bcast", Var, 0},
		{"IPv4len", Const, 0},
		{"IPv4zero", Var, 0},
		{"IPv6interfacelocalallnodes", Var, 0},

pkg net, func FileConn(*os.File) (Conn, error)
pkg net, func FileListener(*os.File) (Listener, error)
pkg net, func FilePacketConn(*os.File) (PacketConn, error)
pkg net, func IPv4(uint8, uint8, uint8, uint8) IP
pkg net, func IPv4Mask(uint8, uint8, uint8, uint8) IPMask
pkg net, func InterfaceAddrs() ([]Addr, error)
pkg net, func InterfaceByIndex(int) (*Interface, error)
pkg net, func InterfaceByName(string) (*Interface, error)