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Network Working Group                                            A. Main
Internet-Draft: draft-main-ipaddr-text-rep-02              Black Ops Ltd
Category: Standards-Track                                     2005-02-23
Expires: 2005-08-23

           Textual Representation of IPv4 and IPv6 Addresses

Status of this Memo

   This document is an Internet-Draft.

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at

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   Historically, the conventional textual representations of IPv4 and
   IPv6 addresses have been poorly specified.  This document gives
   precise definitions of these conventions, together with advice for

Change Log

   Changes from -01 to -02:

   o  Updated reference to what is now RFC 3986.

   Changes from -00 to -01:

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   o  Discussion of the ABNF grammar for IPv4 addresses given in [MTP]
      and [SMTP-1].

   o  Explanation of the faulty ABNF grammar in [IPV6-AA-2].

   o  Mention of another faulty grammar for IPv6 addresses in [SMTP-2].

   o  Specific references for the enclosing of IP addresses in brackets.

   o  Changed intended category from Informational to Standards-Track.

   o  A couple of minor wording changes.

   o  Updated author's address.

1 Introduction

   For as long as IP has existed, there has been a need to represent IP
   addresses in textual contexts, but the nature of these requirements
   has changed.  IP addresses are textually represented much more widely
   than appears to have been originally envisioned; in particular, such
   representation has become a part of many network protocols.  There is
   an increasing need for interoperability in IP address textual
   representations, for it is more commonly software than humans that
   read and write addresses in this format.

   Historically, the definitions of IP address textual representations
   have been loose, underspecifying the syntax.  They have also always
   been a minor part of a standard whose main focus is some other
   problem.  This makes them difficult to locate and inconvenient to
   cite.  With IPv6 address textual representation incorporating the
   IPv4 format by reference, the IPv6 format has not previously been
   completely specified in a single RFC.

   This document collects together the complete syntax for textual
   representation of IPv4 and IPv6 addresses, clarifying the
   underspecified parts.  It is intended to be a complete and
   unambiguous specification of these address formats, located together
   in a single document for ease of reference.

   Section 2 of this document discusses the history of the specification
   and implementation of textual representation of IP addresses.
   Section 3 gives the complete syntax.  Section 4 gives some advice for

1.1 Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",

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   "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
   interpreted as described in [REQ-TERM].

1.2 Augmented BNF Notation

   Syntax specifications in this document use augmented BNF notation as
   defined in [ABNF].  The `core rules' in appendix A of [ABNF] are used
   as defined there.

2 History

2.1 IPv4 Dotted Octet Format

2.1.1 Early Practice

   The original IPv4 "dotted octet" format was never fully defined in
   any RFC, so it is necessary to look at usage, rather than merely find
   an authoritative definition, to determine what the effective syntax
   was.  The first mention of dotted octets in the RFC series is in
   [MTP], a predecessor of SMTP, which interestingly mentions two
   address formats that evidently by then had some currency:

        One form is a decimal integer prefixed by a pound sign, "#",
        which indicates the number is the address of the host.  Another
        form is four small decimal integers separated by dots and
        enclosed by brackets, e.g., "[]", which indicates
        a 32 bit ARPA Internet Address in four eight bit fields.

   [MTP] also gives a partial ABNF specification for these address
   formats.  The grammar requires four dot-separated parts, each of
   which consists of "three digits representing an integer value in the
   range 0 through 255".  Presumably the exclusion of one- and two-digit
   parts is a mistake, since it clashes with the example quoted above.
   [MTP]'s descendent [SMTP-1] gives the same partial ABNF but requires
   each part to consist of "one, two, or three digits representing a
   decimal integer value in the range 0 through 255".

   A few months later, [IPV4-NUMB] (the "Assigned Numbers" RFC published
   at the same time as [IPV4]) gave, for the first time, a table of
   assigned IP addresses.  (Previous "Assigned Numbers" RFCs, predating
   classful addressing, had merely had a table of "network numbers".
   Although the new table retained the title "assigned network numbers",
   it was actually expressed in terms of address blocks.)  This table
   used dotted decimal format, zero-filling each encoded octet to three
   digits.  The notes accompanying the table said:

        One notation for internet host addresses commonly used divides
        the 32-bit address into four 8-bit fields and specifies the

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        value of each field as a decimal number with the fields
        separated by periods.  For example, the internet address of ISIF
        is  This notation will be used in the listing
        of assigned network numbers.

   Shortly thereafter, [NCP-TCP] gave a handful of live IP addresses
   without comment on the format, for example, "ARPANET/SATNET gateway
   at BBN (".

   The next description of dotted octet notation is in [HOST-TBL-2],
   defining the host table file format, which describes the notation as
   "four decimal numbers separated by a period.  Each decimal number
   represents 1 octet.".  One of its example host table entries was
   "GATEWAY :, : MIT-GW :: MOS : IP/GW :".

   It is at this point in the history that [SMTP-1] reiterated the
   formal grammar given in [MTP], with the emendation to unambiguously
   allow decimal numbers of fewer than three digits.

   [HREQ-APP], a much later and more significant standard, describes IP
   address text representation, in the course of recommending that
   applications allow users to specify IP addresses directly as well as
   via DNS host names.  It merely describes the format as "dotted-
   decimal ("#.#.#.#") form".  It gives no example of an address in this

   So far we have seen dotted octet format in five different types of
   situation: a network protocol (machine-parsed email address), a table
   of address blocks, English text (discussion the NCP to TCP/IP
   switch), a machine-readable database (the host table), and human
   interfaces to network applications.  All are consistent about
   dividing the address into octets and representing each octet purely
   in decimal, but there are two variants of the format due to a more
   subtle issue.  The explicit descriptions of the format given so far
   have been silent about the permissibility of leading zeroes in octet
   representations; only one example, a human-oriented table of
   addresses, used leading zeroes.

   This variation in the format, presumably initially intended to be of
   no consequence, lives on today.  The direct descendent of
   [IPV4-NUMB]'s "assigned network numbers" table is the IANA-maintained
   "ipv4-address-space" table, which at the date of this document still
   shows octet values in zero-filled three-digit decimal.  In all non-
   table contexts in which IPv4 addresses appear, including anything
   intended to be machine-readable, almost universally leading zeroes
   are suppressed.  (Curiously, a different IANA-maintained table, the
   "multicast-addresses" table of IPv4 multicast addresses, uses a
   mixture of zero-filled and zero-suppressed octet values.)

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   Meanwhile, a very popular implementation of IP networking went off in
   its own direction.  4.2BSD introduced a function inet_aton(), whose
   job was to interpret character strings as IP addresses.  It
   interpreted both of the syntaxes mentioned in [MTP] (see above): a
   single number giving the entire 32-bit address, and dot-separated
   octet values.  It also interpreted two intermediate syntaxes: octet-
   dot-octet-dot-16bits, intended for class B addresses, and octet-
   dot-24bits, intended for class A addresses.  It also allowed some
   flexibility in how the individual numeric parts were specified: it
   allowed octal and hexadecimal in addition to decimal, distinguishing
   these radices by using the C language syntax involving a prefix "0"
   or "0x", and allowed the numbers to be arbitrarily long.

   The 4.2BSD inet_aton() has been widely copied and imitated, and so is
   a de facto standard for the textual representation of IPv4 addresses.
   Nevertheless, these alternative syntaxes have now fallen out of use
   (if they ever had significant use).  The only practical use that they
   now see is for deliberate obfuscation of addresses: giving an IPv4
   address as a single 32-bit decimal number is favoured among people
   wishing to conceal the true location that is encoded in a URL.  All
   the forms except for decimal octets are seen as non-standard (despite
   being quite widely interoperable) and undesirable.

2.1.2 Revision From IPv6 Work

   When the textual format for IPv6 addresses was developed, part of the
   syntax involved representing an embedded IPv4 address by embedding an
   IPv4 address textual representation in the IPv6 textual format.
   [IPV6-AA-1], describing the IPv6 format for the first time, referred
   simply to "decimal values of the four low-order 8-bit pieces of the
   address (standard IPv4 representation)", giving "::" as an
   example of the format in practice.

   [IPV6-AA-2] added an ABNF grammar, giving the first formal
   specification of IPv4 textual address syntax in the RFC series.  This
   grammar showed dot-separated segments of one to three decimal digits
   each.  Unfortunately, there were some errors in related bits of the
   grammar, and even with errors corrected the IPv6 address grammar was
   loose, syntactically permitting addresses of the wrong length.  This,
   together with the similar looseness of the IPv4 address grammar
   (which would match "123.456.789.999"), left open the question of
   whether the grammar's acceptance of leading zeroes in IPv4 addresses
   was an intentional feature, an error, or deliberate looseness.
   [IPV6-AA-3], rather than correct the errors, withdrew the grammar.

   The IPv6 effort also had an opportunity to advance the other branch
   of development of IPv4 address representation.  [BSI-IPV6-1] doesn't
   attempt to modify inet_aton(), but defines a new function

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   inet_pton(), which, in handling IPv4 addresses, accepts dotted
   decimal octets where each octet is encoded as "a one to three digit
   decimal number between 0 and 255".  The variant forms traditionally
   accepted by inet_aton() are explicitly excluded.  This definition is
   still not explicit about the handling of leading zeroes, but it seems
   to be intended to allow them, and it is being implemented

2.1.3 Finale

   So far we've seen two parallel versions of IPv4 address textual
   syntax, which we may label the IETF version and the BSD version.  The
   difference has persisted for so long because the two are just
   sufficiently interoperable: they both handle in the same way the
   overwhelmingly dominant syntax, dotted decimal octets with leading
   zeroes suppressed.  In all the other address forms they support they
   disagree: the IETF syntax makes nothing of most of the variants that
   BSD allows, and the two interpret differently a large group of
   representations involving leading zeroes, which is why zeroes have
   been mentioned so much in the foregoing history.

   As of this writing, IPv4 addresses written with leading zeroes are de
   facto ambiguous.  Although all IETF output that expresses an opinion
   has consistently indicated that these should be interpreted as
   decimal, implementations that interpret them as octal are far too
   widespread to ignore.  For this reason it is not safe to generate
   such addresses; the only way to generate an interoperable textual
   IPv4 address is to suppress leading zeroes.  Overwhelmingly popular
   practice is, indeed, to avoid leading zeroes.

   The most recent version of the URI syntax [URI] attempts to reconcile
   these variants in order to give a precise definition for acceptable
   IP address syntax in a URL.  (Its predecessors had incorporated the
   traditionally ambiguous syntax by reference.)  [URI] is the first RFC
   to require a completely rigorous definition of IP address syntax.
   The approach taken was to standardise the safe common subset of the
   IETF and BSD syntaxes, which achieves standardisation on IETF-like
   syntax while also retaining backward compatibility with existing BSD-
   based implementations.

   This document, in section 3.1, presents the IPv4 address grammar from

2.2 IPv6 Presentation Format

   The development of the IPv6 address presentation format has been
   simpler than the IPv4 history.  The divergence between specification
   and implementation has been less significant, and there has been

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   conscious effort to fully specify the format rather than leave it as
   oral tradition.

   The first appearance of IPv6 address textual format in the RFC series
   is the specification of the format in [IPV6-AA-1].  This
   specification's relevant features are: a basic format of eight colon-
   separated 16-bit pieces; each piece represented in hexadecimal, with
   leading zeroes "not necessary" (examples are given both with and
   without leading zeroes); optional use of "::", once in an address, to
   indicate a run of zero-valued 16-bit pieces; optional use of
   "standard IPv4 representation" for the least-significant 32 bits of
   the address.

   Note that this doesn't say what the maximum length of a piece
   representation is, or whether "::" can be used in an address where
   all 16-bit pieces are given explicitly (the "::" would represent a
   sequence of zero consecutive zero-valued pieces).

   [IPV6-AA-2] didn't substantially modify the description of the
   syntax, but augmented it with an ABNF grammar.  The grammar specified
   that a 16-bit piece could be represented in one to four case-
   insensitive hexadecimal digits, ruling out the use of more than four
   digits per piece.  There were some errors in the grammar, making it
   inappropriate as a reference, and some looseness that makes it
   impossible to clear up any other syntactic uncertainty from it.
   These errors resulted from a "rough cut" grammar getting into the RFC
   largely unchanged, clearly without serious analysis.

   [SMTP-2] gives an ABNF grammar for IPv6 address literals in email
   addresses.  Unfortunately this grammar, also, is faulty and loose,
   but it concurs with the [IPV6-AA-2] grammar in requiring each piece
   to consist of one to four hexadecimal digits.

   [IPV6-AA-3] dropped the faulty grammar from [IPV6-AA-2], and amended
   the format description to say that "::" represents "one or more"
   16-bit pieces.  This amended description leaves unclear only the
   issue of whether a 16-bit piece is permitted to be written with more
   than four hexadecimal digits; fortunately the intended answer (which
   is that it is not permitted) is known from the [IPV6-AA-2] and
   [SMTP-2] ABNF grammars.  This document, in section 3.2, presents this

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3 Syntax and Semantics

3.1 IPv4 Dotted Octet Format

   A 32-bit IPv4 address is divided into four octets.  Each octet is
   represented numerically in decimal, using the minimum possible number
   of digits (leading zeroes are not used, except in the case of 0
   itself).  The four encoded octets are given most-significant first,
   separated by period characters.

        IPv4address = d8 "." d8 "." d8 "." d8

        d8          = DIGIT               ; 0-9
                    / %x31-39 DIGIT       ; 10-99
                    / "1" 2DIGIT          ; 100-199
                    / "2" %x30-34 DIGIT   ; 200-249
                    / "25" %x30-35        ; 250-255

3.2 IPv6 Presentation Format

   A 128-bit IPv6 address is divided into eight 16-bit pieces.  Each
   piece is represented numerically in case-insensitive hexadecimal,
   using one to four hexadecimal digits (leading zeroes are permitted).
   The eight encoded pieces are given most-significant first, separated
   by colon characters.  Optionally, the least-significant two pieces
   may instead be represented in IPv4 address textual format (the
   <IPv4address> production given above).  Optionally, once in the
   address, a sequence of one or more consecutive zero-valued 16-bit
   pieces may be elided, omitting all their digits and leaving exactly
   two consecutive colons in their place to mark the elision.

        IPv6address =                          6(h16 ":") ls32
                    /                     "::" 5(h16 ":") ls32
                    / [             h16 ] "::" 4(h16 ":") ls32
                    / [ *1(h16 ":") h16 ] "::" 3(h16 ":") ls32
                    / [ *2(h16 ":") h16 ] "::" 2(h16 ":") ls32
                    / [ *3(h16 ":") h16 ] "::"   h16 ":"  ls32
                    / [ *4(h16 ":") h16 ] "::"            ls32
                    / [ *5(h16 ":") h16 ] "::"             h16
                    / [ *6(h16 ":") h16 ] "::"

        ls32        = h16 ":" h16 / IPv4address

        h16         = 1*4HEXDIG

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4 Recommendations

4.1 Be Stringent in What You Accept

   Interpreting textual network addresses is a case where being liberal
   in what one receives is not a virtue.  In addition to the well-known
   problem of interoperability testing against a liberal implementation
   leading to insufficiently conservative sending behaviour, variations
   on the address syntaxes tend to result in strings whose intended
   meaning is unclear.  Since a misinterpreted network address is quite
   useless, whereas partial misinterpretation of most other things is
   forgivable, it is particularly important to reject any address whose
   interpretation is in question.

   For backward compatibility, some applications will wish to continue
   supporting some of the variations discussed in section 2.  New
   applications, however, SHOULD accept only the syntax given in section
   3.  Regardless of any alternative syntax that is supported, the
   standard syntax given in section 3 MUST be interpreted exactly as
   described there.

4.2 Generation of Representations of IPv6 Addresses

   The standard format for IPv6 addresses has several options, granting
   some discretion in the choice of representation.  The choices
   available are:

   o  which case to use for hexadecimal digits above 9;

   o  whether to use leading zeroes in the representation of 16-bit
      pieces whose upper four bits are all zero;

   o  whether to represent the least-significant 32 bits as two pieces
      in hexadecimal or in IPv4 format;

   o  whether to elide a sequence of zero-valued pieces, and which such
      sequence to elide.

   For specific applications there may be needs that dictate some of
   these choices.  For example, if laying out IPv6 addresses vertically
   in a table, comparison is eased by using a fixed format by including
   all leading zeroes and not eliding zero-valued pieces.

   For general-purpose use, common practice is to use lowercase, use
   nearly the shortest possible representation, and to represent
   IPv4-compatible and IPv4-mapped addresses using the embedded IPv4
   address representation.  Experience has shown this format to be
   nearly optimal for human comprehension of an address presented in

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   isolation, and so is RECOMMENDED when there are no strong
   considerations promoting a different format.  To generate this

   o  Use the embedded IPv4 address format for addresses in
      ::ffff:0:0/96 (IPv4-mapped addresses), and in ::/96
      (IPv4-compatible addresses) except for :: (the unspecified
      address) and ::1 (the loopback address) which are not
      IPv4-compatible addresses.

   o  Omit all optional leading zeroes in the representations of 16-bit

   o  If there are any sequences of consecutive zero-valued pieces,
      elide the longest such sequence.  In case of a tie, it seems to be
      most common to pick the leftmost candidate.

4.3 Delimitation

   Textually-represented IPv4 and IPv6 addresses have a sufficiently
   narrow format that delimitation is rarely a problem.  In human-
   readable text they look sufficiently like words that additional
   delimitation is usually not required; adjacent punctuation mostly
   wouldn't be a valid character in the address, and even with
   punctuation that can appear in the addresses (period and colon)
   trailing punctuation creates no ambiguity due to the restricted use
   of punctuation in the addresses.

   A significant area where there is a delimitation issue is when an IP
   address is presented together with an alphanumeric subaddress such as
   a TCP port number.  Some applications separate an IP address and port
   number using a period, which, particularly in the case of IPv4, makes
   the port number visually appear to be part of the address.  This is
   particularly tricky to read if a bare IP address without port number
   might appear in the same context.  Some applications use a colon to
   separate IP address and port number, which is good for IPv4 but in
   IPv6 it creates the same kind of problem that the period did in IPv4,
   and can actually give an ambiguous result if a bare IPv6 address is
   permitted in the same context.  Applications SHOULD, therefore, pick
   some other character to separate IP addresses and port numbers; BIND,
   for example, uses "#".  "/" is not recommended, due to a clash with
   address prefix syntax.

   In contexts where an IP address needs to be distinguished from
   similar-looking data that can appear in the same place, there is
   precedent for enclosing an IP address in brackets ("[]") as a
   distinguisher.  Precedents are email addresses [SMTP-2] and URIs

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5 Security Considerations

   In a network protocol, representation of network addresses in a
   textual format raises no inherent issues over representation in a
   binary format.  Care should be taken to ensure that textual addresses
   are parsed safely, so that bad syntax will not cause unwanted
   behaviour.  Where a textually-represented address is expected, it
   should be decoded by a subroutine that will decode only the expected
   address format and will not do anything (besides report an error) if
   given some other input such as a host name.

   In applications, the capability for the user to specify a network
   node by address as well as by name is both powerful and potentially
   dangerous.  If an application does not intend to let the user specify
   absolutely any network resource, then it should either have only a
   more restrictive means of identifying network nodes or apply
   reasonableness checks on the address that the user enters.

6 Acknowledgements

   This document is a spin-off from the development of [URI], which was
   the first RFC to give such a precise definition of IP address textual
   syntax as is given here.  The ABNF rules in section 3 were developed
   collaboratively by Roy T. Fielding (author of [URI]) and the author
   of this document.

   Brian Carpenter confessed to being the perpetrator of the faulty
   grammar that got into [IPV6-AA-2], and supplied a copy of the
   historically-interesting email message in which he originally
   proposed it.  He also made some useful comments.

7 Normative References

   [ABNF]       D. Crocker, Ed., P. Overell, "Augmented BNF for Syntax
                Specifications: ABNF", RFC 2234, November 1997.

   [REQ-TERM]   S. Bradner, "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

8 Informative References

   [BSI-IPV6-1] R. Gilligan, S. Thomson, J. Bound, W. Stevens, "Basic
                Socket Interface Extensions for IPv6", RFC 2133, April

   [HOST-TBL-2] E.J. Feinler, K. Harrenstien, Z. Su, V. White, "DoD
                Internet host table specification", RFC 810,

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   [HREQ-APP]   R.T. Braden, "Requirements for Internet hosts -
                application and support", STD 3, RFC 1123, Oct-01-1989.

   [IPV4]       J. Postel, "Internet Protocol", RFC 791, Sep-01-1981.

   [IPV4-NUMB]  J. Postel, "Assigned numbers", RFC 790, Sep-01-1981.

   [IPV6-AA-1]  R. Hinden, S. Deering, Eds., "IP Version 6 Addressing
                Architecture", RFC 1884, December 1995.

   [IPV6-AA-2]  R. Hinden, S. Deering, "IP Version 6 Addressing
                Architecture", RFC 2373, July 1998.

   [IPV6-AA-3]  R. Hinden, S. Deering, "Internet Protocol Version 6
                (IPv6) Addressing Architecture", RFC 3513, April 2003.

   [MTP]        S. Sluizer, J. Postel, "Mail Transfer Protocol", RFC
                780, May-01-1981.

   [NCP-TCP]    J. Postel, "NCP/TCP transition plan", RFC 801,

   [SMTP-1]     J. Postel, "Simple Mail Transfer Protocol", STD 10, RFC
                821, Aug-01-1982.

   [SMTP-2]     J. Klensin, Ed., "Simple Mail Transfer Protocol", RFC
                2821, April 2001.

   [URI]        T. Berners-Lee, R. Fielding, L. Masinter, "Uniform
                Resource Identifier (URI): Generic Syntax", RFC 3986,
                January 2005.

9 Author's Address

   Andrew Main
   Black Ops Ltd
   Flat 2
   84 Isledon Road
   N7 7JS
   United Kingdom

   Phone: +44 7887 945779
   EMail: zefram@fysh.org

10 Copyright and Disclaimer

   Copyright (C) The Internet Society 2005. This document is subject

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   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an

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