Source file src/net/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 IP mask is an IP address.
    35  type IPMask []byte
    36  
    37  // An IPNet represents an IP network.
    38  type IPNet struct {
    39  	IP   IP     // network number
    40  	Mask IPMask // network mask
    41  }
    42  
    43  // IPv4 returns the IP address (in 16-byte form) of the
    44  // IPv4 address a.b.c.d.
    45  func IPv4(a, b, c, d byte) IP {
    46  	p := make(IP, IPv6len)
    47  	copy(p, v4InV6Prefix)
    48  	p[12] = a
    49  	p[13] = b
    50  	p[14] = c
    51  	p[15] = d
    52  	return p
    53  }
    54  
    55  var v4InV6Prefix = []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff}
    56  
    57  // IPv4Mask returns the IP mask (in 4-byte form) of the
    58  // IPv4 mask a.b.c.d.
    59  func IPv4Mask(a, b, c, d byte) IPMask {
    60  	p := make(IPMask, IPv4len)
    61  	p[0] = a
    62  	p[1] = b
    63  	p[2] = c
    64  	p[3] = d
    65  	return p
    66  }
    67  
    68  // CIDRMask returns an IPMask consisting of `ones' 1 bits
    69  // followed by 0s up to a total length of `bits' bits.
    70  // For a mask of this form, CIDRMask is the inverse of IPMask.Size.
    71  func CIDRMask(ones, bits int) IPMask {
    72  	if bits != 8*IPv4len && bits != 8*IPv6len {
    73  		return nil
    74  	}
    75  	if ones < 0 || ones > bits {
    76  		return nil
    77  	}
    78  	l := bits / 8
    79  	m := make(IPMask, l)
    80  	n := uint(ones)
    81  	for i := 0; i < l; i++ {
    82  		if n >= 8 {
    83  			m[i] = 0xff
    84  			n -= 8
    85  			continue
    86  		}
    87  		m[i] = ^byte(0xff >> n)
    88  		n = 0
    89  	}
    90  	return m
    91  }
    92  
    93  // Well-known IPv4 addresses
    94  var (
    95  	IPv4bcast     = IPv4(255, 255, 255, 255) // limited broadcast
    96  	IPv4allsys    = IPv4(224, 0, 0, 1)       // all systems
    97  	IPv4allrouter = IPv4(224, 0, 0, 2)       // all routers
    98  	IPv4zero      = IPv4(0, 0, 0, 0)         // all zeros
    99  )
   100  
   101  // Well-known IPv6 addresses
   102  var (
   103  	IPv6zero                   = IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
   104  	IPv6unspecified            = IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
   105  	IPv6loopback               = IP{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1}
   106  	IPv6interfacelocalallnodes = IP{0xff, 0x01, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x01}
   107  	IPv6linklocalallnodes      = IP{0xff, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x01}
   108  	IPv6linklocalallrouters    = IP{0xff, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x02}
   109  )
   110  
   111  // IsUnspecified reports whether ip is an unspecified address, either
   112  // the IPv4 address "0.0.0.0" or the IPv6 address "::".
   113  func (ip IP) IsUnspecified() bool {
   114  	return ip.Equal(IPv4zero) || ip.Equal(IPv6unspecified)
   115  }
   116  
   117  // IsLoopback reports whether ip is a loopback address.
   118  func (ip IP) IsLoopback() bool {
   119  	if ip4 := ip.To4(); ip4 != nil {
   120  		return ip4[0] == 127
   121  	}
   122  	return ip.Equal(IPv6loopback)
   123  }
   124  
   125  // IsMulticast reports whether ip is a multicast address.
   126  func (ip IP) IsMulticast() bool {
   127  	if ip4 := ip.To4(); ip4 != nil {
   128  		return ip4[0]&0xf0 == 0xe0
   129  	}
   130  	return len(ip) == IPv6len && ip[0] == 0xff
   131  }
   132  
   133  // IsInterfaceLocalMulticast reports whether ip is
   134  // an interface-local multicast address.
   135  func (ip IP) IsInterfaceLocalMulticast() bool {
   136  	return len(ip) == IPv6len && ip[0] == 0xff && ip[1]&0x0f == 0x01
   137  }
   138  
   139  // IsLinkLocalMulticast reports whether ip is a link-local
   140  // multicast address.
   141  func (ip IP) IsLinkLocalMulticast() bool {
   142  	if ip4 := ip.To4(); ip4 != nil {
   143  		return ip4[0] == 224 && ip4[1] == 0 && ip4[2] == 0
   144  	}
   145  	return len(ip) == IPv6len && ip[0] == 0xff && ip[1]&0x0f == 0x02
   146  }
   147  
   148  // IsLinkLocalUnicast reports whether ip is a link-local
   149  // unicast address.
   150  func (ip IP) IsLinkLocalUnicast() bool {
   151  	if ip4 := ip.To4(); ip4 != nil {
   152  		return ip4[0] == 169 && ip4[1] == 254
   153  	}
   154  	return len(ip) == IPv6len && ip[0] == 0xfe && ip[1]&0xc0 == 0x80
   155  }
   156  
   157  // IsGlobalUnicast reports whether ip is a global unicast
   158  // address.
   159  //
   160  // The identification of global unicast addresses uses address type
   161  // identification as defined in RFC 1122, RFC 4632 and RFC 4291 with
   162  // the exception of IPv4 directed broadcast addresses.
   163  // It returns true even if ip is in IPv4 private address space or
   164  // local IPv6 unicast address space.
   165  func (ip IP) IsGlobalUnicast() bool {
   166  	return (len(ip) == IPv4len || len(ip) == IPv6len) &&
   167  		!ip.Equal(IPv4bcast) &&
   168  		!ip.IsUnspecified() &&
   169  		!ip.IsLoopback() &&
   170  		!ip.IsMulticast() &&
   171  		!ip.IsLinkLocalUnicast()
   172  }
   173  
   174  // Is p all zeros?
   175  func isZeros(p IP) bool {
   176  	for i := 0; i < len(p); i++ {
   177  		if p[i] != 0 {
   178  			return false
   179  		}
   180  	}
   181  	return true
   182  }
   183  
   184  // To4 converts the IPv4 address ip to a 4-byte representation.
   185  // If ip is not an IPv4 address, To4 returns nil.
   186  func (ip IP) To4() IP {
   187  	if len(ip) == IPv4len {
   188  		return ip
   189  	}
   190  	if len(ip) == IPv6len &&
   191  		isZeros(ip[0:10]) &&
   192  		ip[10] == 0xff &&
   193  		ip[11] == 0xff {
   194  		return ip[12:16]
   195  	}
   196  	return nil
   197  }
   198  
   199  // To16 converts the IP address ip to a 16-byte representation.
   200  // If ip is not an IP address (it is the wrong length), To16 returns nil.
   201  func (ip IP) To16() IP {
   202  	if len(ip) == IPv4len {
   203  		return IPv4(ip[0], ip[1], ip[2], ip[3])
   204  	}
   205  	if len(ip) == IPv6len {
   206  		return ip
   207  	}
   208  	return nil
   209  }
   210  
   211  // Default route masks for IPv4.
   212  var (
   213  	classAMask = IPv4Mask(0xff, 0, 0, 0)
   214  	classBMask = IPv4Mask(0xff, 0xff, 0, 0)
   215  	classCMask = IPv4Mask(0xff, 0xff, 0xff, 0)
   216  )
   217  
   218  // DefaultMask returns the default IP mask for the IP address ip.
   219  // Only IPv4 addresses have default masks; DefaultMask returns
   220  // nil if ip is not a valid IPv4 address.
   221  func (ip IP) DefaultMask() IPMask {
   222  	if ip = ip.To4(); ip == nil {
   223  		return nil
   224  	}
   225  	switch {
   226  	case ip[0] < 0x80:
   227  		return classAMask
   228  	case ip[0] < 0xC0:
   229  		return classBMask
   230  	default:
   231  		return classCMask
   232  	}
   233  }
   234  
   235  func allFF(b []byte) bool {
   236  	for _, c := range b {
   237  		if c != 0xff {
   238  			return false
   239  		}
   240  	}
   241  	return true
   242  }
   243  
   244  // Mask returns the result of masking the IP address ip with mask.
   245  func (ip IP) Mask(mask IPMask) IP {
   246  	if len(mask) == IPv6len && len(ip) == IPv4len && allFF(mask[:12]) {
   247  		mask = mask[12:]
   248  	}
   249  	if len(mask) == IPv4len && len(ip) == IPv6len && bytealg.Equal(ip[:12], v4InV6Prefix) {
   250  		ip = ip[12:]
   251  	}
   252  	n := len(ip)
   253  	if n != len(mask) {
   254  		return nil
   255  	}
   256  	out := make(IP, n)
   257  	for i := 0; i < n; i++ {
   258  		out[i] = ip[i] & mask[i]
   259  	}
   260  	return out
   261  }
   262  
   263  // ubtoa encodes the string form of the integer v to dst[start:] and
   264  // returns the number of bytes written to dst. The caller must ensure
   265  // that dst has sufficient length.
   266  func ubtoa(dst []byte, start int, v byte) int {
   267  	if v < 10 {
   268  		dst[start] = v + '0'
   269  		return 1
   270  	} else if v < 100 {
   271  		dst[start+1] = v%10 + '0'
   272  		dst[start] = v/10 + '0'
   273  		return 2
   274  	}
   275  
   276  	dst[start+2] = v%10 + '0'
   277  	dst[start+1] = (v/10)%10 + '0'
   278  	dst[start] = v/100 + '0'
   279  	return 3
   280  }
   281  
   282  // String returns the string form of the IP address ip.
   283  // It returns one of 4 forms:
   284  //   - "<nil>", if ip has length 0
   285  //   - dotted decimal ("192.0.2.1"), if ip is an IPv4 or IP4-mapped IPv6 address
   286  //   - IPv6 ("2001:db8::1"), if ip is a valid IPv6 address
   287  //   - the hexadecimal form of ip, without punctuation, if no other cases apply
   288  func (ip IP) String() string {
   289  	p := ip
   290  
   291  	if len(ip) == 0 {
   292  		return "<nil>"
   293  	}
   294  
   295  	// If IPv4, use dotted notation.
   296  	if p4 := p.To4(); len(p4) == IPv4len {
   297  		const maxIPv4StringLen = len("255.255.255.255")
   298  		b := make([]byte, maxIPv4StringLen)
   299  
   300  		n := ubtoa(b, 0, p4[0])
   301  		b[n] = '.'
   302  		n++
   303  
   304  		n += ubtoa(b, n, p4[1])
   305  		b[n] = '.'
   306  		n++
   307  
   308  		n += ubtoa(b, n, p4[2])
   309  		b[n] = '.'
   310  		n++
   311  
   312  		n += ubtoa(b, n, p4[3])
   313  		return string(b[:n])
   314  	}
   315  	if len(p) != IPv6len {
   316  		return "?" + hexString(ip)
   317  	}
   318  
   319  	// Find longest run of zeros.
   320  	e0 := -1
   321  	e1 := -1
   322  	for i := 0; i < IPv6len; i += 2 {
   323  		j := i
   324  		for j < IPv6len && p[j] == 0 && p[j+1] == 0 {
   325  			j += 2
   326  		}
   327  		if j > i && j-i > e1-e0 {
   328  			e0 = i
   329  			e1 = j
   330  			i = j
   331  		}
   332  	}
   333  	// The symbol "::" MUST NOT be used to shorten just one 16 bit 0 field.
   334  	if e1-e0 <= 2 {
   335  		e0 = -1
   336  		e1 = -1
   337  	}
   338  
   339  	const maxLen = len("ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff")
   340  	b := make([]byte, 0, maxLen)
   341  
   342  	// Print with possible :: in place of run of zeros
   343  	for i := 0; i < IPv6len; i += 2 {
   344  		if i == e0 {
   345  			b = append(b, ':', ':')
   346  			i = e1
   347  			if i >= IPv6len {
   348  				break
   349  			}
   350  		} else if i > 0 {
   351  			b = append(b, ':')
   352  		}
   353  		b = appendHex(b, (uint32(p[i])<<8)|uint32(p[i+1]))
   354  	}
   355  	return string(b)
   356  }
   357  
   358  func hexString(b []byte) string {
   359  	s := make([]byte, len(b)*2)
   360  	for i, tn := range b {
   361  		s[i*2], s[i*2+1] = hexDigit[tn>>4], hexDigit[tn&0xf]
   362  	}
   363  	return string(s)
   364  }
   365  
   366  // ipEmptyString is like ip.String except that it returns
   367  // an empty string when ip is unset.
   368  func ipEmptyString(ip IP) string {
   369  	if len(ip) == 0 {
   370  		return ""
   371  	}
   372  	return ip.String()
   373  }
   374  
   375  // MarshalText implements the encoding.TextMarshaler interface.
   376  // The encoding is the same as returned by String, with one exception:
   377  // When len(ip) is zero, it returns an empty slice.
   378  func (ip IP) MarshalText() ([]byte, error) {
   379  	if len(ip) == 0 {
   380  		return []byte(""), nil
   381  	}
   382  	if len(ip) != IPv4len && len(ip) != IPv6len {
   383  		return nil, &AddrError{Err: "invalid IP address", Addr: hexString(ip)}
   384  	}
   385  	return []byte(ip.String()), nil
   386  }
   387  
   388  // UnmarshalText implements the encoding.TextUnmarshaler interface.
   389  // The IP address is expected in a form accepted by ParseIP.
   390  func (ip *IP) UnmarshalText(text []byte) error {
   391  	if len(text) == 0 {
   392  		*ip = nil
   393  		return nil
   394  	}
   395  	s := string(text)
   396  	x := ParseIP(s)
   397  	if x == nil {
   398  		return &ParseError{Type: "IP address", Text: s}
   399  	}
   400  	*ip = x
   401  	return nil
   402  }
   403  
   404  // Equal reports whether ip and x are the same IP address.
   405  // An IPv4 address and that same address in IPv6 form are
   406  // considered to be equal.
   407  func (ip IP) Equal(x IP) bool {
   408  	if len(ip) == len(x) {
   409  		return bytealg.Equal(ip, x)
   410  	}
   411  	if len(ip) == IPv4len && len(x) == IPv6len {
   412  		return bytealg.Equal(x[0:12], v4InV6Prefix) && bytealg.Equal(ip, x[12:])
   413  	}
   414  	if len(ip) == IPv6len && len(x) == IPv4len {
   415  		return bytealg.Equal(ip[0:12], v4InV6Prefix) && bytealg.Equal(ip[12:], x)
   416  	}
   417  	return false
   418  }
   419  
   420  func (ip IP) matchAddrFamily(x IP) bool {
   421  	return ip.To4() != nil && x.To4() != nil || ip.To16() != nil && ip.To4() == nil && x.To16() != nil && x.To4() == nil
   422  }
   423  
   424  // If mask is a sequence of 1 bits followed by 0 bits,
   425  // return the number of 1 bits.
   426  func simpleMaskLength(mask IPMask) int {
   427  	var n int
   428  	for i, v := range mask {
   429  		if v == 0xff {
   430  			n += 8
   431  			continue
   432  		}
   433  		// found non-ff byte
   434  		// count 1 bits
   435  		for v&0x80 != 0 {
   436  			n++
   437  			v <<= 1
   438  		}
   439  		// rest must be 0 bits
   440  		if v != 0 {
   441  			return -1
   442  		}
   443  		for i++; i < len(mask); i++ {
   444  			if mask[i] != 0 {
   445  				return -1
   446  			}
   447  		}
   448  		break
   449  	}
   450  	return n
   451  }
   452  
   453  // Size returns the number of leading ones and total bits in the mask.
   454  // If the mask is not in the canonical form--ones followed by zeros--then
   455  // Size returns 0, 0.
   456  func (m IPMask) Size() (ones, bits int) {
   457  	ones, bits = simpleMaskLength(m), len(m)*8
   458  	if ones == -1 {
   459  		return 0, 0
   460  	}
   461  	return
   462  }
   463  
   464  // String returns the hexadecimal form of m, with no punctuation.
   465  func (m IPMask) String() string {
   466  	if len(m) == 0 {
   467  		return "<nil>"
   468  	}
   469  	return hexString(m)
   470  }
   471  
   472  func networkNumberAndMask(n *IPNet) (ip IP, m IPMask) {
   473  	if ip = n.IP.To4(); ip == nil {
   474  		ip = n.IP
   475  		if len(ip) != IPv6len {
   476  			return nil, nil
   477  		}
   478  	}
   479  	m = n.Mask
   480  	switch len(m) {
   481  	case IPv4len:
   482  		if len(ip) != IPv4len {
   483  			return nil, nil
   484  		}
   485  	case IPv6len:
   486  		if len(ip) == IPv4len {
   487  			m = m[12:]
   488  		}
   489  	default:
   490  		return nil, nil
   491  	}
   492  	return
   493  }
   494  
   495  // Contains reports whether the network includes ip.
   496  func (n *IPNet) Contains(ip IP) bool {
   497  	nn, m := networkNumberAndMask(n)
   498  	if x := ip.To4(); x != nil {
   499  		ip = x
   500  	}
   501  	l := len(ip)
   502  	if l != len(nn) {
   503  		return false
   504  	}
   505  	for i := 0; i < l; i++ {
   506  		if nn[i]&m[i] != ip[i]&m[i] {
   507  			return false
   508  		}
   509  	}
   510  	return true
   511  }
   512  
   513  // Network returns the address's network name, "ip+net".
   514  func (n *IPNet) Network() string { return "ip+net" }
   515  
   516  // String returns the CIDR notation of n like "192.0.2.1/24"
   517  // or "2001:db8::/48" as defined in RFC 4632 and RFC 4291.
   518  // If the mask is not in the canonical form, it returns the
   519  // string which consists of an IP address, followed by a slash
   520  // character and a mask expressed as hexadecimal form with no
   521  // punctuation like "198.51.100.1/c000ff00".
   522  func (n *IPNet) String() string {
   523  	nn, m := networkNumberAndMask(n)
   524  	if nn == nil || m == nil {
   525  		return "<nil>"
   526  	}
   527  	l := simpleMaskLength(m)
   528  	if l == -1 {
   529  		return nn.String() + "/" + m.String()
   530  	}
   531  	return nn.String() + "/" + uitoa(uint(l))
   532  }
   533  
   534  // Parse IPv4 address (d.d.d.d).
   535  func parseIPv4(s string) IP {
   536  	var p [IPv4len]byte
   537  	for i := 0; i < IPv4len; i++ {
   538  		if len(s) == 0 {
   539  			// Missing octets.
   540  			return nil
   541  		}
   542  		if i > 0 {
   543  			if s[0] != '.' {
   544  				return nil
   545  			}
   546  			s = s[1:]
   547  		}
   548  		n, c, ok := dtoi(s)
   549  		if !ok || n > 0xFF {
   550  			return nil
   551  		}
   552  		s = s[c:]
   553  		p[i] = byte(n)
   554  	}
   555  	if len(s) != 0 {
   556  		return nil
   557  	}
   558  	return IPv4(p[0], p[1], p[2], p[3])
   559  }
   560  
   561  // parseIPv6Zone parses s as a literal IPv6 address and its associated zone
   562  // identifier which is described in RFC 4007.
   563  func parseIPv6Zone(s string) (IP, string) {
   564  	s, zone := splitHostZone(s)
   565  	return parseIPv6(s), zone
   566  }
   567  
   568  // parseIPv6Zone parses s as a literal IPv6 address described in RFC 4291
   569  // and RFC 5952.
   570  func parseIPv6(s string) (ip IP) {
   571  	ip = make(IP, IPv6len)
   572  	ellipsis := -1 // position of ellipsis in ip
   573  
   574  	// Might have leading ellipsis
   575  	if len(s) >= 2 && s[0] == ':' && s[1] == ':' {
   576  		ellipsis = 0
   577  		s = s[2:]
   578  		// Might be only ellipsis
   579  		if len(s) == 0 {
   580  			return ip
   581  		}
   582  	}
   583  
   584  	// Loop, parsing hex numbers followed by colon.
   585  	i := 0
   586  	for i < IPv6len {
   587  		// Hex number.
   588  		n, c, ok := xtoi(s)
   589  		if !ok || n > 0xFFFF {
   590  			return nil
   591  		}
   592  
   593  		// If followed by dot, might be in trailing IPv4.
   594  		if c < len(s) && s[c] == '.' {
   595  			if ellipsis < 0 && i != IPv6len-IPv4len {
   596  				// Not the right place.
   597  				return nil
   598  			}
   599  			if i+IPv4len > IPv6len {
   600  				// Not enough room.
   601  				return nil
   602  			}
   603  			ip4 := parseIPv4(s)
   604  			if ip4 == nil {
   605  				return nil
   606  			}
   607  			ip[i] = ip4[12]
   608  			ip[i+1] = ip4[13]
   609  			ip[i+2] = ip4[14]
   610  			ip[i+3] = ip4[15]
   611  			s = ""
   612  			i += IPv4len
   613  			break
   614  		}
   615  
   616  		// Save this 16-bit chunk.
   617  		ip[i] = byte(n >> 8)
   618  		ip[i+1] = byte(n)
   619  		i += 2
   620  
   621  		// Stop at end of string.
   622  		s = s[c:]
   623  		if len(s) == 0 {
   624  			break
   625  		}
   626  
   627  		// Otherwise must be followed by colon and more.
   628  		if s[0] != ':' || len(s) == 1 {
   629  			return nil
   630  		}
   631  		s = s[1:]
   632  
   633  		// Look for ellipsis.
   634  		if s[0] == ':' {
   635  			if ellipsis >= 0 { // already have one
   636  				return nil
   637  			}
   638  			ellipsis = i
   639  			s = s[1:]
   640  			if len(s) == 0 { // can be at end
   641  				break
   642  			}
   643  		}
   644  	}
   645  
   646  	// Must have used entire string.
   647  	if len(s) != 0 {
   648  		return nil
   649  	}
   650  
   651  	// If didn't parse enough, expand ellipsis.
   652  	if i < IPv6len {
   653  		if ellipsis < 0 {
   654  			return nil
   655  		}
   656  		n := IPv6len - i
   657  		for j := i - 1; j >= ellipsis; j-- {
   658  			ip[j+n] = ip[j]
   659  		}
   660  		for j := ellipsis + n - 1; j >= ellipsis; j-- {
   661  			ip[j] = 0
   662  		}
   663  	} else if ellipsis >= 0 {
   664  		// Ellipsis must represent at least one 0 group.
   665  		return nil
   666  	}
   667  	return ip
   668  }
   669  
   670  // ParseIP parses s as an IP address, returning the result.
   671  // The string s can be in dotted decimal ("192.0.2.1")
   672  // or IPv6 ("2001:db8::68") form.
   673  // If s is not a valid textual representation of an IP address,
   674  // ParseIP returns nil.
   675  func ParseIP(s string) IP {
   676  	for i := 0; i < len(s); i++ {
   677  		switch s[i] {
   678  		case '.':
   679  			return parseIPv4(s)
   680  		case ':':
   681  			return parseIPv6(s)
   682  		}
   683  	}
   684  	return nil
   685  }
   686  
   687  // parseIPZone parses s as an IP address, return it and its associated zone
   688  // identifier (IPv6 only).
   689  func parseIPZone(s string) (IP, string) {
   690  	for i := 0; i < len(s); i++ {
   691  		switch s[i] {
   692  		case '.':
   693  			return parseIPv4(s), ""
   694  		case ':':
   695  			return parseIPv6Zone(s)
   696  		}
   697  	}
   698  	return nil, ""
   699  }
   700  
   701  // ParseCIDR parses s as a CIDR notation IP address and prefix length,
   702  // like "192.0.2.0/24" or "2001:db8::/32", as defined in
   703  // RFC 4632 and RFC 4291.
   704  //
   705  // It returns the IP address and the network implied by the IP and
   706  // prefix length.
   707  // For example, ParseCIDR("192.0.2.1/24") returns the IP address
   708  // 192.0.2.1 and the network 192.0.2.0/24.
   709  func ParseCIDR(s string) (IP, *IPNet, error) {
   710  	i := bytealg.IndexByteString(s, '/')
   711  	if i < 0 {
   712  		return nil, nil, &ParseError{Type: "CIDR address", Text: s}
   713  	}
   714  	addr, mask := s[:i], s[i+1:]
   715  	iplen := IPv4len
   716  	ip := parseIPv4(addr)
   717  	if ip == nil {
   718  		iplen = IPv6len
   719  		ip = parseIPv6(addr)
   720  	}
   721  	n, i, ok := dtoi(mask)
   722  	if ip == nil || !ok || i != len(mask) || n < 0 || n > 8*iplen {
   723  		return nil, nil, &ParseError{Type: "CIDR address", Text: s}
   724  	}
   725  	m := CIDRMask(n, 8*iplen)
   726  	return ip, &IPNet{IP: ip.Mask(m), Mask: m}, nil
   727  }
   728  

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