ip.go

Documentation: net

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // IP address manipulations
     6  //
     7  // IPv4 addresses are 4 bytes; IPv6 addresses are 16 bytes.
     8  // An IPv4 address can be converted to an IPv6 address by
     9  // adding a canonical prefix (10 zeros, 2 0xFFs).
    10  // This library accepts either size of byte slice but always
    11  // returns 16-byte addresses.
    12  
    13  package net
    14  
    15  import "internal/bytealg"
    16  
    17  // IP address lengths (bytes).
    18  const (
    19  	IPv4len = 4
    20  	IPv6len = 16
    21  )
    22  
    23  // An IP is a single IP address, a slice of bytes.
    24  // Functions in this package accept either 4-byte (IPv4)
    25  // or 16-byte (IPv6) slices as input.
    26  //
    27  // Note that in this documentation, referring to an
    28  // IP address as an IPv4 address or an IPv6 address
    29  // is a semantic property of the address, not just the
    30  // length of the byte slice: a 16-byte slice can still
    31  // be an IPv4 address.
    32  type IP []byte
    33  
    34  // An IPMask is a bitmask that can be used to manipulate
    35  // IP addresses for IP addressing and routing.
    36  //
    37  // See type IPNet and func ParseCIDR for details.
    38  type IPMask []byte
    39  
    40  // An IPNet represents an IP network.
    41  type IPNet struct {
    42  	IP   IP     // network number
    43  	Mask IPMask // network mask
    44  }
    45  
    46  // IPv4 returns the IP address (in 16-byte form) of the
    47  // IPv4 address a.b.c.d.
    48  func IPv4(a, b, c, d byte) IP {
    49  	p := make(IP, IPv6len)
    50  	copy(p, v4InV6Prefix)
    51  	p[12] = a
    52  	p[13] = b
    53  	p[14] = c
    54  	p[15] = d
    55  	return p
    56  }
    57  
    58  var v4InV6Prefix = []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff}
    59  
    60  // IPv4Mask returns the IP mask (in 4-byte form) of the
    61  // IPv4 mask a.b.c.d.
    62  func IPv4Mask(a, b, c, d byte) IPMask {
    63  	p := make(IPMask, IPv4len)
    64  	p[0] = a
    65  	p[1] = b
    66  	p[2] = c
    67  	p[3] = d
    68  	return p
    69  }
    70  
    71  // CIDRMask returns an IPMask consisting of 'ones' 1 bits
    72  // followed by 0s up to a total length of 'bits' bits.
    73  // For a mask of this form, CIDRMask is the inverse of IPMask.Size.
    74  func CIDRMask(ones, bits int) IPMask {
    75  	if bits != 8*IPv4len && bits != 8*IPv6len {
    76  		return nil
    77  	}
    78  	if ones < 0 || ones > bits {
    79  		return nil
    80  	}
    81  	l := bits / 8
    82  	m := make(IPMask, l)
    83  	n := uint(ones)
    84  	for i := 0; i < l; i++ {
    85  		if n >= 8 {
    86  			m[i] = 0xff
    87  			n -= 8
    88  			continue
    89  		}
    90  		m[i] = ^byte(0xff >> n)
    91  		n = 0
    92  	}
    93  	return m
    94  }
    95  
    96  // Well-known IPv4 addresses
    97  var (
    98  	IPv4bcast     = IPv4(255, 255, 255, 255) // limited broadcast
    99  	IPv4allsys    = IPv4(224, 0, 0, 1)       // all systems
   100  	IPv4allrouter = IPv4(224, 0, 0, 2)       // all routers
   101  	IPv4zero      = IPv4(0, 0, 0, 0)         // all zeros
   102  )
   103  
   104  // Well-known IPv6 addresses
   105  var (
   106  	IPv6zero                   = IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
   107  	IPv6unspecified            = IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
   108  	IPv6loopback               = IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1}
   109  	IPv6interfacelocalallnodes = IP{0xff, 0x01, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x01}
   110  	IPv6linklocalallnodes      = IP{0xff, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x01}
   111  	IPv6linklocalallrouters    = IP{0xff, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x02}
   112  )
   113  
   114  // IsUnspecified reports whether ip is an unspecified address, either
   115  // the IPv4 address "0.0.0.0" or the IPv6 address "::".
   116  func (ip IP) IsUnspecified() bool {
   117  	return ip.Equal(IPv4zero) || ip.Equal(IPv6unspecified)
   118  }
   119  
   120  // IsLoopback reports whether ip is a loopback address.
   121  func (ip IP) IsLoopback() bool {
   122  	if ip4 := ip.To4(); ip4 != nil {
   123  		return ip4[0] == 127
   124  	}
   125  	return ip.Equal(IPv6loopback)
   126  }
   127  
   128  // IsMulticast reports whether ip is a multicast address.
   129  func (ip IP) IsMulticast() bool {
   130  	if ip4 := ip.To4(); ip4 != nil {
   131  		return ip4[0]&0xf0 == 0xe0
   132  	}
   133  	return len(ip) == IPv6len && ip[0] == 0xff
   134  }
   135  
   136  // IsInterfaceLocalMulticast reports whether ip is
   137  // an interface-local multicast address.
   138  func (ip IP) IsInterfaceLocalMulticast() bool {
   139  	return len(ip) == IPv6len && ip[0] == 0xff && ip[1]&0x0f == 0x01
   140  }
   141  
   142  // IsLinkLocalMulticast reports whether ip is a link-local
   143  // multicast address.
   144  func (ip IP) IsLinkLocalMulticast() bool {
   145  	if ip4 := ip.To4(); ip4 != nil {
   146  		return ip4[0] == 224 && ip4[1] == 0 && ip4[2] == 0
   147  	}
   148  	return len(ip) == IPv6len && ip[0] == 0xff && ip[1]&0x0f == 0x02
   149  }
   150  
   151  // IsLinkLocalUnicast reports whether ip is a link-local
   152  // unicast address.
   153  func (ip IP) IsLinkLocalUnicast() bool {
   154  	if ip4 := ip.To4(); ip4 != nil {
   155  		return ip4[0] == 169 && ip4[1] == 254
   156  	}
   157  	return len(ip) == IPv6len && ip[0] == 0xfe && ip[1]&0xc0 == 0x80
   158  }
   159  
   160  // IsGlobalUnicast reports whether ip is a global unicast
   161  // address.
   162  //
   163  // The identification of global unicast addresses uses address type
   164  // identification as defined in RFC 1122, RFC 4632 and RFC 4291 with
   165  // the exception of IPv4 directed broadcast addresses.
   166  // It returns true even if ip is in IPv4 private address space or
   167  // local IPv6 unicast address space.
   168  func (ip IP) IsGlobalUnicast() bool {
   169  	return (len(ip) == IPv4len || len(ip) == IPv6len) &&
   170  		!ip.Equal(IPv4bcast) &&
   171  		!ip.IsUnspecified() &&
   172  		!ip.IsLoopback() &&
   173  		!ip.IsMulticast() &&
   174  		!ip.IsLinkLocalUnicast()
   175  }
   176  
   177  // Is p all zeros?
   178  func isZeros(p IP) bool {
   179  	for i := 0; i < len(p); i++ {
   180  		if p[i] != 0 {
   181  			return false
   182  		}
   183  	}
   184  	return true
   185  }
   186  
   187  // To4 converts the IPv4 address ip to a 4-byte representation.
   188  // If ip is not an IPv4 address, To4 returns nil.
   189  func (ip IP) To4() IP {
   190  	if len(ip) == IPv4len {
   191  		return ip
   192  	}
   193  	if len(ip) == IPv6len &&
   194  		isZeros(ip[0:10]) &&
   195  		ip[10] == 0xff &&
   196  		ip[11] == 0xff {
   197  		return ip[12:16]
   198  	}
   199  	return nil
   200  }
   201  
   202  // To16 converts the IP address ip to a 16-byte representation.
   203  // If ip is not an IP address (it is the wrong length), To16 returns nil.
   204  func (ip IP) To16() IP {
   205  	if len(ip) == IPv4len {
   206  		return IPv4(ip[0], ip[1], ip[2], ip[3])
   207  	}
   208  	if len(ip) == IPv6len {
   209  		return ip
   210  	}
   211  	return nil
   212  }
   213  
   214  // Default route masks for IPv4.
   215  var (
   216  	classAMask = IPv4Mask(0xff, 0, 0, 0)
   217  	classBMask = IPv4Mask(0xff, 0xff, 0, 0)
   218  	classCMask = IPv4Mask(0xff, 0xff, 0xff, 0)
   219  )
   220  
   221  // DefaultMask returns the default IP mask for the IP address ip.
   222  // Only IPv4 addresses have default masks; DefaultMask returns
   223  // nil if ip is not a valid IPv4 address.
   224  func (ip IP) DefaultMask() IPMask {
   225  	if ip = ip.To4(); ip == nil {
   226  		return nil
   227  	}
   228  	switch {
   229  	case ip[0] < 0x80:
   230  		return classAMask
   231  	case ip[0] < 0xC0:
   232  		return classBMask
   233  	default:
   234  		return classCMask
   235  	}
   236  }
   237  
   238  func allFF(b []byte) bool {
   239  	for _, c := range b {
   240  		if c != 0xff {
   241  			return false
   242  		}
   243  	}
   244  	return true
   245  }
   246  
   247  // Mask returns the result of masking the IP address ip with mask.
   248  func (ip IP) Mask(mask IPMask) IP {
   249  	if len(mask) == IPv6len && len(ip) == IPv4len && allFF(mask[:12]) {
   250  		mask = mask[12:]
   251  	}
   252  	if len(mask) == IPv4len && len(ip) == IPv6len && bytealg.Equal(ip[:12], v4InV6Prefix) {
   253  		ip = ip[12:]
   254  	}
   255  	n := len(ip)
   256  	if n != len(mask) {
   257  		return nil
   258  	}
   259  	out := make(IP, n)
   260  	for i := 0; i < n; i++ {
   261  		out[i] = ip[i] & mask[i]
   262  	}
   263  	return out
   264  }
   265  
   266  // ubtoa encodes the string form of the integer v to dst[start:] and
   267  // returns the number of bytes written to dst. The caller must ensure
   268  // that dst has sufficient length.
   269  func ubtoa(dst []byte, start int, v byte) int {
   270  	if v < 10 {
   271  		dst[start] = v + '0'
   272  		return 1
   273  	} else if v < 100 {
   274  		dst[start+1] = v%10 + '0'
   275  		dst[start] = v/10 + '0'
   276  		return 2
   277  	}
   278  
   279  	dst[start+2] = v%10 + '0'
   280  	dst[start+1] = (v/10)%10 + '0'
   281  	dst[start] = v/100 + '0'
   282  	return 3
   283  }
   284  
   285  // String returns the string form of the IP address ip.
   286  // It returns one of 4 forms:
   287  //   - "<nil>", if ip has length 0
   288  //   - dotted decimal ("192.0.2.1"), if ip is an IPv4 or IP4-mapped IPv6 address
   289  //   - IPv6 ("2001:db8::1"), if ip is a valid IPv6 address
   290  //   - the hexadecimal form of ip, without punctuation, if no other cases apply
   291  func (ip IP) String() string {
   292  	p := ip
   293  
   294  	if len(ip) == 0 {
   295  		return "<nil>"
   296  	}
   297  
   298  	// If IPv4, use dotted notation.
   299  	if p4 := p.To4(); len(p4) == IPv4len {
   300  		const maxIPv4StringLen = len("255.255.255.255")
   301  		b := make([]byte, maxIPv4StringLen)
   302  
   303  		n := ubtoa(b, 0, p4[0])
   304  		b[n] = '.'
   305  		n++
   306  
   307  		n += ubtoa(b, n, p4[1])
   308  		b[n] = '.'
   309  		n++
   310  
   311  		n += ubtoa(b, n, p4[2])
   312  		b[n] = '.'
   313  		n++
   314  
   315  		n += ubtoa(b, n, p4[3])
   316  		return string(b[:n])
   317  	}
   318  	if len(p) != IPv6len {
   319  		return "?" + hexString(ip)
   320  	}
   321  
   322  	// Find longest run of zeros.
   323  	e0 := -1
   324  	e1 := -1
   325  	for i := 0; i < IPv6len; i += 2 {
   326  		j := i
   327  		for j < IPv6len && p[j] == 0 && p[j+1] == 0 {
   328  			j += 2
   329  		}
   330  		if j > i && j-i > e1-e0 {
   331  			e0 = i
   332  			e1 = j
   333  			i = j
   334  		}
   335  	}
   336  	// The symbol "::" MUST NOT be used to shorten just one 16 bit 0 field.
   337  	if e1-e0 <= 2 {
   338  		e0 = -1
   339  		e1 = -1
   340  	}
   341  
   342  	const maxLen = len("ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff")
   343  	b := make([]byte, 0, maxLen)
   344  
   345  	// Print with possible :: in place of run of zeros
   346  	for i := 0; i < IPv6len; i += 2 {
   347  		if i == e0 {
   348  			b = append(b, ':', ':')
   349  			i = e1
   350  			if i >= IPv6len {
   351  				break
   352  			}
   353  		} else if i > 0 {
   354  			b = append(b, ':')
   355  		}
   356  		b = appendHex(b, (uint32(p[i])<<8)|uint32(p[i+1]))
   357  	}
   358  	return string(b)
   359  }
   360  
   361  func hexString(b []byte) string {
   362  	s := make([]byte, len(b)*2)
   363  	for i, tn := range b {
   364  		s[i*2], s[i*2+1] = hexDigit[tn>>4], hexDigit[tn&0xf]
   365  	}
   366  	return string(s)
   367  }
   368  
   369  // ipEmptyString is like ip.String except that it returns
   370  // an empty string when ip is unset.
   371  func ipEmptyString(ip IP) string {
   372  	if len(ip) == 0 {
   373  		return ""
   374  	}
   375  	return ip.String()
   376  }
   377  
   378  // MarshalText implements the encoding.TextMarshaler interface.
   379  // The encoding is the same as returned by String, with one exception:
   380  // When len(ip) is zero, it returns an empty slice.
   381  func (ip IP) MarshalText() ([]byte, error) {
   382  	if len(ip) == 0 {
   383  		return []byte(""), nil
   384  	}
   385  	if len(ip) != IPv4len && len(ip) != IPv6len {
   386  		return nil, &AddrError{Err: "invalid IP address", Addr: hexString(ip)}
   387  	}
   388  	return []byte(ip.String()), nil
   389  }
   390  
   391  // UnmarshalText implements the encoding.TextUnmarshaler interface.
   392  // The IP address is expected in a form accepted by ParseIP.
   393  func (ip *IP) UnmarshalText(text []byte) error {
   394  	if len(text) == 0 {
   395  		*ip = nil
   396  		return nil
   397  	}
   398  	s := string(text)
   399  	x := ParseIP(s)
   400  	if x == nil {
   401  		return &ParseError{Type: "IP address", Text: s}
   402  	}
   403  	*ip = x
   404  	return nil
   405  }
   406  
   407  // Equal reports whether ip and x are the same IP address.
   408  // An IPv4 address and that same address in IPv6 form are
   409  // considered to be equal.
   410  func (ip IP) Equal(x IP) bool {
   411  	if len(ip) == len(x) {
   412  		return bytealg.Equal(ip, x)
   413  	}
   414  	if len(ip) == IPv4len && len(x) == IPv6len {
   415  		return bytealg.Equal(x[0:12], v4InV6Prefix) && bytealg.Equal(ip, x[12:])
   416  	}
   417  	if len(ip) == IPv6len && len(x) == IPv4len {
   418  		return bytealg.Equal(ip[0:12], v4InV6Prefix) && bytealg.Equal(ip[12:], x)
   419  	}
   420  	return false
   421  }
   422  
   423  func (ip IP) matchAddrFamily(x IP) bool {
   424  	return ip.To4() != nil && x.To4() != nil || ip.To16() != nil && ip.To4() == nil && x.To16() != nil && x.To4() == nil
   425  }
   426  
   427  // If mask is a sequence of 1 bits followed by 0 bits,
   428  // return the number of 1 bits.
   429  func simpleMaskLength(mask IPMask) int {
   430  	var n int
   431  	for i, v := range mask {
   432  		if v == 0xff {
   433  			n += 8
   434  			continue
   435  		}
   436  		// found non-ff byte
   437  		// count 1 bits
   438  		for v&0x80 != 0 {
   439  			n++
   440  			v <<= 1
   441  		}
   442  		// rest must be 0 bits
   443  		if v != 0 {
   444  			return -1
   445  		}
   446  		for i++; i < len(mask); i++ {
   447  			if mask[i] != 0 {
   448  				return -1
   449  			}
   450  		}
   451  		break
   452  	}
   453  	return n
   454  }
   455  
   456  // Size returns the number of leading ones and total bits in the mask.
   457  // If the mask is not in the canonical form--ones followed by zeros--then
   458  // Size returns 0, 0.
   459  func (m IPMask) Size() (ones, bits int) {
   460  	ones, bits = simpleMaskLength(m), len(m)*8
   461  	if ones == -1 {
   462  		return 0, 0
   463  	}
   464  	return
   465  }
   466  
   467  // String returns the hexadecimal form of m, with no punctuation.
   468  func (m IPMask) String() string {
   469  	if len(m) == 0 {
   470  		return "<nil>"
   471  	}
   472  	return hexString(m)
   473  }
   474  
   475  func networkNumberAndMask(n *IPNet) (ip IP, m IPMask) {
   476  	if ip = n.IP.To4(); ip == nil {
   477  		ip = n.IP
   478  		if len(ip) != IPv6len {
   479  			return nil, nil
   480  		}
   481  	}
   482  	m = n.Mask
   483  	switch len(m) {
   484  	case IPv4len:
   485  		if len(ip) != IPv4len {
   486  			return nil, nil
   487  		}
   488  	case IPv6len:
   489  		if len(ip) == IPv4len {
   490  			m = m[12:]
   491  		}
   492  	default:
   493  		return nil, nil
   494  	}
   495  	return
   496  }
   497  
   498  // Contains reports whether the network includes ip.
   499  func (n *IPNet) Contains(ip IP) bool {
   500  	nn, m := networkNumberAndMask(n)
   501  	if x := ip.To4(); x != nil {
   502  		ip = x
   503  	}
   504  	l := len(ip)
   505  	if l != len(nn) {
   506  		return false
   507  	}
   508  	for i := 0; i < l; i++ {
   509  		if nn[i]&m[i] != ip[i]&m[i] {
   510  			return false
   511  		}
   512  	}
   513  	return true
   514  }
   515  
   516  // Network returns the address's network name, "ip+net".
   517  func (n *IPNet) Network() string { return "ip+net" }
   518  
   519  // String returns the CIDR notation of n like "192.0.2.0/24"
   520  // or "2001:db8::/48" as defined in RFC 4632 and RFC 4291.
   521  // If the mask is not in the canonical form, it returns the
   522  // string which consists of an IP address, followed by a slash
   523  // character and a mask expressed as hexadecimal form with no
   524  // punctuation like "198.51.100.0/c000ff00".
   525  func (n *IPNet) String() string {
   526  	nn, m := networkNumberAndMask(n)
   527  	if nn == nil || m == nil {
   528  		return "<nil>"
   529  	}
   530  	l := simpleMaskLength(m)
   531  	if l == -1 {
   532  		return nn.String() + "/" + m.String()
   533  	}
   534  	return nn.String() + "/" + uitoa(uint(l))
   535  }
   536  
   537  // Parse IPv4 address (d.d.d.d).
   538  func parseIPv4(s string) IP {
   539  	var p [IPv4len]byte
   540  	for i := 0; i < IPv4len; i++ {
   541  		if len(s) == 0 {
   542  			// Missing octets.
   543  			return nil
   544  		}
   545  		if i > 0 {
   546  			if s[0] != '.' {
   547  				return nil
   548  			}
   549  			s = s[1:]
   550  		}
   551  		n, c, ok := dtoi(s)
   552  		if !ok || n > 0xFF {
   553  			return nil
   554  		}
   555  		s = s[c:]
   556  		p[i] = byte(n)
   557  	}
   558  	if len(s) != 0 {
   559  		return nil
   560  	}
   561  	return IPv4(p[0], p[1], p[2], p[3])
   562  }
   563  
   564  // parseIPv6Zone parses s as a literal IPv6 address and its associated zone
   565  // identifier which is described in RFC 4007.
   566  func parseIPv6Zone(s string) (IP, string) {
   567  	s, zone := splitHostZone(s)
   568  	return parseIPv6(s), zone
   569  }
   570  
   571  // parseIPv6 parses s as a literal IPv6 address described in RFC 4291
   572  // and RFC 5952.
   573  func parseIPv6(s string) (ip IP) {
   574  	ip = make(IP, IPv6len)
   575  	ellipsis := -1 // position of ellipsis in ip
   576  
   577  	// Might have leading ellipsis
   578  	if len(s) >= 2 && s[0] == ':' && s[1] == ':' {
   579  		ellipsis = 0
   580  		s = s[2:]
   581  		// Might be only ellipsis
   582  		if len(s) == 0 {
   583  			return ip
   584  		}
   585  	}
   586  
   587  	// Loop, parsing hex numbers followed by colon.
   588  	i := 0
   589  	for i < IPv6len {
   590  		// Hex number.
   591  		n, c, ok := xtoi(s)
   592  		if !ok || n > 0xFFFF {
   593  			return nil
   594  		}
   595  
   596  		// If followed by dot, might be in trailing IPv4.
   597  		if c < len(s) && s[c] == '.' {
   598  			if ellipsis < 0 && i != IPv6len-IPv4len {
   599  				// Not the right place.
   600  				return nil
   601  			}
   602  			if i+IPv4len > IPv6len {
   603  				// Not enough room.
   604  				return nil
   605  			}
   606  			ip4 := parseIPv4(s)
   607  			if ip4 == nil {
   608  				return nil
   609  			}
   610  			ip[i] = ip4[12]
   611  			ip[i+1] = ip4[13]
   612  			ip[i+2] = ip4[14]
   613  			ip[i+3] = ip4[15]
   614  			s = ""
   615  			i += IPv4len
   616  			break
   617  		}
   618  
   619  		// Save this 16-bit chunk.
   620  		ip[i] = byte(n >> 8)
   621  		ip[i+1] = byte(n)
   622  		i += 2
   623  
   624  		// Stop at end of string.
   625  		s = s[c:]
   626  		if len(s) == 0 {
   627  			break
   628  		}
   629  
   630  		// Otherwise must be followed by colon and more.
   631  		if s[0] != ':' || len(s) == 1 {
   632  			return nil
   633  		}
   634  		s = s[1:]
   635  
   636  		// Look for ellipsis.
   637  		if s[0] == ':' {
   638  			if ellipsis >= 0 { // already have one
   639  				return nil
   640  			}
   641  			ellipsis = i
   642  			s = s[1:]
   643  			if len(s) == 0 { // can be at end
   644  				break
   645  			}
   646  		}
   647  	}
   648  
   649  	// Must have used entire string.
   650  	if len(s) != 0 {
   651  		return nil
   652  	}
   653  
   654  	// If didn't parse enough, expand ellipsis.
   655  	if i < IPv6len {
   656  		if ellipsis < 0 {
   657  			return nil
   658  		}
   659  		n := IPv6len - i
   660  		for j := i - 1; j >= ellipsis; j-- {
   661  			ip[j+n] = ip[j]
   662  		}
   663  		for j := ellipsis + n - 1; j >= ellipsis; j-- {
   664  			ip[j] = 0
   665  		}
   666  	} else if ellipsis >= 0 {
   667  		// Ellipsis must represent at least one 0 group.
   668  		return nil
   669  	}
   670  	return ip
   671  }
   672  
   673  // ParseIP parses s as an IP address, returning the result.
   674  // The string s can be in IPv4 dotted decimal ("192.0.2.1"), IPv6
   675  // ("2001:db8::68"), or IPv4-mapped IPv6 ("::ffff:192.0.2.1") form.
   676  // If s is not a valid textual representation of an IP address,
   677  // ParseIP returns nil.
   678  func ParseIP(s string) IP {
   679  	for i := 0; i < len(s); i++ {
   680  		switch s[i] {
   681  		case '.':
   682  			return parseIPv4(s)
   683  		case ':':
   684  			return parseIPv6(s)
   685  		}
   686  	}
   687  	return nil
   688  }
   689  
   690  // parseIPZone parses s as an IP address, return it and its associated zone
   691  // identifier (IPv6 only).
   692  func parseIPZone(s string) (IP, string) {
   693  	for i := 0; i < len(s); i++ {
   694  		switch s[i] {
   695  		case '.':
   696  			return parseIPv4(s), ""
   697  		case ':':
   698  			return parseIPv6Zone(s)
   699  		}
   700  	}
   701  	return nil, ""
   702  }
   703  
   704  // ParseCIDR parses s as a CIDR notation IP address and prefix length,
   705  // like "192.0.2.0/24" or "2001:db8::/32", as defined in
   706  // RFC 4632 and RFC 4291.
   707  //
   708  // It returns the IP address and the network implied by the IP and
   709  // prefix length.
   710  // For example, ParseCIDR("192.0.2.1/24") returns the IP address
   711  // 192.0.2.1 and the network 192.0.2.0/24.
   712  func ParseCIDR(s string) (IP, *IPNet, error) {
   713  	i := bytealg.IndexByteString(s, '/')
   714  	if i < 0 {
   715  		return nil, nil, &ParseError{Type: "CIDR address", Text: s}
   716  	}
   717  	addr, mask := s[:i], s[i+1:]
   718  	iplen := IPv4len
   719  	ip := parseIPv4(addr)
   720  	if ip == nil {
   721  		iplen = IPv6len
   722  		ip = parseIPv6(addr)
   723  	}
   724  	n, i, ok := dtoi(mask)
   725  	if ip == nil || !ok || i != len(mask) || n < 0 || n > 8*iplen {
   726  		return nil, nil, &ParseError{Type: "CIDR address", Text: s}
   727  	}
   728  	m := CIDRMask(n, 8*iplen)
   729  	return ip, &IPNet{IP: ip.Mask(m), Mask: m}, nil
   730  }
   731
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