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Network Working Group P. Mockapetris Request for Comments: 1101 ISI Updates: RFCs 1034, 1035 April 1989

           DNS Encoding of Network Names and Other Types


 This RFC proposes two extensions to the Domain Name System:
  1. A specific method for entering and retrieving RRs which map

between network names and numbers.

  1. Ideas for a general method for describing mappings between

arbitrary identifiers and numbers.

 The method for mapping between network names and addresses is a
 proposed standard, the ideas for a general method are experimental.
 This RFC assumes that the reader is familiar with the DNS [RFC 1034,
 RFC 1035] and its use.  The data shown is for pedagogical use and
 does not necessarily reflect the real Internet.
 Distribution of this memo is unlimited.


 The DNS is extensible and can be used for a virtually unlimited
 number of data types, name spaces, etc.  New type definitions are
 occasionally necessary as are revisions or deletions of old types
 (e.g., MX replacement of MD and MF [RFC 974]), and changes described
 in [RFC 973].  This RFC describes changes due to the general need to
 map between identifiers and values, and a specific need for network
 name support.
 Users wish to be able to use the DNS to map between network names and
 numbers.  This need is the only capability found in HOSTS.TXT which
 is not available from the DNS.  In designing a method to do this,
 there were two major areas of concern:
  1. Several tradeoffs involving control of network names, the

syntax of network names, backward compatibility, etc.

  1. A desire to create a method which would be sufficiently

general to set a good precedent for future mappings,

      for example, between TCP-port names and numbers,

Mockapetris [Page 1] RFC 1101 DNS Encoding of Network Names and Other Types April 1989

      autonomous system names and numbers, X.500 Relative
      Distinguished Names (RDNs) and their servers, or whatever.
 It was impossible to reconcile these two areas of concern for network
 names because of the desire to unify network number support within
 existing IP address to host name support.  The existing support is
 the IN-ADDR.ARPA section of the DNS name space.  As a result this RFC
 describes one structure for network names which builds on the
 existing support for host names, and another family of structures for
 future yellow pages (YP) functions such as conversions between TCP-
 port numbers and mnemonics.
 Both structures are described in following sections.  Each structure
 has a discussion of design issues and specific structure
 We wish to avoid defining structures and methods which can work but
 do not because of indifference or errors on the part of system
 administrators when maintaining the database.  The WKS RR is an
 example.  Thus, while we favor distribution as a general method, we
 also recognize that centrally maintained tables (such as HOSTS.TXT)
 are usually more consistent though less maintainable and timely.
 Hence we recommend both specific methods for mapping network names,
 addresses, and subnets, as well as an instance of the general method
 for mapping between allocated network numbers and network names.
 (Allocation is centrally performed by the SRI Network Information
 Center, aka the NIC).


 The issues involved in the design were the definition of network name
 syntax, the mappings to be provided, and possible support for similar
 functions at the subnet level.

3.1. Network name syntax

 The current syntax for network names, as defined by [RFC 952] is an
 alphanumeric string of up to 24 characters, which begins with an
 alpha, and may include "." and "-" except as first and last
 characters.  This is the format which was also used for host names
 before the DNS.  Upward compatibility with existing names might be a
 goal of any new scheme.
 However, the present syntax has been used to define a flat name
 space, and hence would prohibit the same distributed name allocation
 method used for host names.  There is some sentiment for allowing the
 NIC to continue to allocate and regulate network names, much as it
 allocates numbers, but the majority opinion favors local control of

Mockapetris [Page 2] RFC 1101 DNS Encoding of Network Names and Other Types April 1989

 network names.  Although it would be possible to provide a flat space
 or a name space in which, for example, the last label of a domain
 name captured the old-style network name, any such approach would add
 complexity to the method and create different rules for network names
 and host names.
 For these reasons, we assume that the syntax of network names will be
 the same as the expanded syntax for host names permitted in [HR].
 The new syntax expands the set of names to allow leading digits, so
 long as the resulting representations do not conflict with IP
 addresses in decimal octet form.  For example, 3Com.COM and 3M.COM
 are now legal, although is not.  See [HR] for details.
 The price is that network names will get as complicated as host
 names.  An administrator will be able to create network names in any
 domain under his control, and also create network number to name
 entries in IN-ADDR.ARPA domains under his control.  Thus, the name
 for the ARPANET might become NET.ARPA, ARPANET.ARPA or Arpa-
 network.MIL., depending on the preferences of the owner.

3.2. Mappings

 The desired mappings, ranked by priority with most important first,
  1. Mapping a IP address or network number to a network name.
      This mapping is for use in debugging tools and status displays
      of various sorts.  The conversion from IP address to network
      number is well known for class A, B, and C IP addresses, and
      involves a simple mask operation.  The needs of other classes
      are not yet defined and are ignored for the rest of this RFC.
  1. Mapping a network name to a network address.
      This facility is of less obvious application, but a
      symmetrical mapping seems desirable.
  1. Mapping an organization to its network names and numbers.
      This facility is useful because it may not always be possible
      to guess the local choice for network names, but the
      organization name is often well known.
  1. Similar mappings for subnets, even when nested.
      The primary application is to be able to identify all of the
      subnets involved in a particular IP address.  A secondary

Mockapetris [Page 3] RFC 1101 DNS Encoding of Network Names and Other Types April 1989

      requirement is to retrieve address mask information.

3.3. Network address section of the name space

 The network name syntax discussed above can provide domain names
 which will contain mappings from network names to various quantities,
 but we also need a section of the name space, organized by network
 and subnet number to hold the inverse mappings.
 The choices include:
  1. The same network number slots already assigned and delegated

in the IN-ADDR.ARPA section of the name space.

      For example, 10.IN-ADDR.ARPA for class A net 10,
      2.128.IN-ADDR.ARPA for class B net 128.2, etc.
  1. Host-zero addresses in the IN-ADDR.ARPA tree. (A host field

of all zero in an IP address is prohibited because of

      confusion related to broadcast addresses, et al.)
      For example, for class A net 10, for class B net 128.2, etc.  Like the
      first scheme, it uses in-place name space delegations to
      distribute control.
      The main advantage of this scheme over the first is that it
      allows convenient names for subnets as well as networks.  A
      secondary advantage is that it uses names which are not in use
      already, and hence it is possible to test whether an
      organization has entered this information in its domain
  1. Some new section of the name space.
      While this option provides the most opportunities, it creates
      a need to delegate a whole new name space.  Since the IP
      address space is so closely related to the network number
      space, most believe that the overhead of creating such a new
      space is overwhelming and would lead to the WKS syndrome.  (As
      of February, 1989, approximately 400 sections of the
      IN-ADDR.ARPA tree are already delegated, usually at network

Mockapetris [Page 4] RFC 1101 DNS Encoding of Network Names and Other Types April 1989


 The proposed solution uses information stored at:
  1. Names in the IN-ADDR.ARPA tree that correspond to host-zero IP

addresses. The same method is used for subnets in a nested

      fashion.  For example, for net 10.
      Two types of information are stored here: PTR RRs which point
      to the network name in their data sections, and A RRs, which
      are present if the network (or subnet) is subnetted further.
      If a type A RR is present, then it has the address mask as its
      data.  The general form is:
      <reversed-host-zero-number>.IN-ADDR.ARPA. PTR <network-name>
      <reversed-host-zero-number>.IN-ADDR.ARPA. A   <subnet-mask>
      For example:  PTR     ARPANET.ARPA.
      or PTR
      In general, this information will be added to an existing
      master file for some IN-ADDR.ARPA domain for each network
      involved.  Similar RRs can be used at host-zero subnet
  1. Names which are network names.
      The data stored here is PTR RRs pointing at the host-zero
      entries.  The general form is:
      <network-name> ptr <reversed-host-zero-number>.IN-ADDR.ARPA
      For example:
      ARPANET.ARPA.           PTR
      or        PTR
      In general, this information will be inserted in the master
      file for the domain name of the organization; this is a

Mockapetris [Page 5] RFC 1101 DNS Encoding of Network Names and Other Types April 1989

      different file from that which holds the information below
      IN-ADDR.ARPA.  Similar PTR RRs can be used at subnet names.
  1. Names corresponding to organizations.
      The data here is one or more PTR RRs pointing at the
      IN-ADDR.ARPA names corresponding to host-zero entries for
      For example:
      ISI.EDU.        PTR
      MCC.COM.        PTR

4.1. A simple example

 The ARPANET is a Class A network without subnets.  The RRs which
 would be added, assuming the ARPANET.ARPA was selected as a network
 name, would be:
 ARPA.                   PTR
 The first RR states that the organization named ARPA owns net 10 (It
 might also own more network numbers, and these would be represented
 with an additional RR per net.)  The second states that the network
 name ARPANET.ARPA. maps to net 10.  The last states that net 10 is
 Note that all of the usual host and corresponding IN-ADDR.ARPA
 entries would still be required.

4.2. A complicated, subnetted example

 The ISI network is 128.9, a class B number.  Suppose the ISI network
 was organized into two levels of subnet, with the first level using
 an additional 8 bits of address, and the second level using 4 bits,
 for address masks of x'FFFFFF00' and X'FFFFFFF0'.
 Then the following RRs would be entered in ISI's master file for the
 ISI.EDU zone:

Mockapetris [Page 6] RFC 1101 DNS Encoding of Network Names and Other Types April 1989

 ; Define network entry                PTR
 ; Define first level subnets            PTR            PTR
 ; Define second level subnets          PTR
 in the 9.128.IN-ADDR.ARPA zone:
 ; Define network number and address mask         PTR
                                 A  ;aka X'FFFFFF00'
 ; Define one of the first level subnet numbers and masks         PTR
                                 A ;aka X'FFFFFFF0'
 ; Define another first level subnet number and mask         PTR
                                 A ;aka X'FFFFFFF0'
 ; Define second level subnet number        PTR
 This assumes that the ISI network is named, first
 level subnets are named and div2-, and a second level subnet is called inc-  (In a real system as complicated as this there
 would be more first and second level subnets defined, but we have
 shown enough to illustrate the ideas.)

4.3. Procedure for using an IP address to get network name

 Depending on whether the IP address is class A, B, or C, mask off the
 high one, two, or three bytes, respectively.  Reverse the octets,
 suffix IN-ADDR.ARPA, and do a PTR query.
 For example, suppose the IP address is
    1. Since this is a class A address, use a mask x'FF000000' and
    2. Construct the name
    3. Do a PTR query.  Get back

Mockapetris [Page 7] RFC 1101 DNS Encoding of Network Names and Other Types April 1989  PTR     ARPANET.ARPA.
    4. Conclude that the network name is "ARPANET.ARPA."
 Suppose that the IP address is
    1. Since this is a class B address, use a mask of x'FFFF0000'
       and get
    2. Construct the name
    3. Do a PTR query.  Get back       PTR
    4. Conclude that the network name is ""

4.4. Procedure for finding all subnets involved with an IP address

 This is a simple extension of the IP address to network name method.
 When the network entry is located, do a lookup for a possible A RR.
 If the A RR is found, look up the next level of subnet using the
 original IP address and the mask in the A RR.  Repeat this procedure
 until no A RR is found.
 For example, repeating the use of
    1. As before construct a query for
       Retrieve:  PTR
    2. Since an A RR was found, repeat using mask from RR
       (, constructing a query for  Retrieve:  PTR
    3. Since another A RR was found, repeat using mask (x'FFFFFFF0').  constructing a query for  Retrieve: PTR
    4. Since no A RR is present at, there
       are no more subnet levels.

Mockapetris [Page 8] RFC 1101 DNS Encoding of Network Names and Other Types April 1989


 The term "Yellow Pages" is used in almost as many ways as the term
 "domain", so it is useful to define what is meant herein by YP.  The
 general problem to be solved is to create a method for creating
 mappings from one kind of identifier to another, often with an
 inverse capability.  The traditional methods are to search or use a
 precomputed index of some kind.
 Searching is impractical when the search is too large, and
 precomputed indexes are possible only when it is possible to specify
 search criteria in advance, and pay for the resources necessary to
 build the index.  For example, it is impractical to search the entire
 domain tree to find a particular address RR, so we build the IN-
 ADDR.ARPA YP.  Similarly, we could never build an Internet-wide index
 of "hosts with a load average of less than 2" in less time than it
 would take for the data to change, so indexes are a useless approach
 for that problem.
 Such a precomputed index is what we mean by YP, and we regard the
 IN-ADDR.ARPA domain as the first instance of a YP in the DNS.
 Although a single, centrally-managed YP for well-known values such as
 TCP-port is desirable, we regard organization-specific YPs for, say,
 locally defined TCP ports as a natural extension, as are combinations
 of YPs using search lists to merge the two.
 In examining Internet Numbers [RFC 997] and Assigned Numbers [RFC
 1010], it is clear that there are several mappings which might be of
 value.  For example:
 <assigned-network-name> <==> <IP-address>
 <autonomous-system-id>  <==> <number>
 <protocol-id>           <==> <number>
 <port-id>               <==> <number>
 <ethernet-type>         <==> <number>
 <public-data-net>       <==> <IP-address>
 Following the IN-ADDR example, the YP takes the form of a domain tree
 organized to optimize retrieval by search key and distribution via
 normal DNS rules.  The name used as a key must include:
    1. A well known origin.  For example, IN-ADDR.ARPA is the
       current IP-address to host name YP.
    2. A "from" data type.  This identifies the input type of the
       mapping.  This is necessary because we may be mapping
       something as anonymous as a number to any number of
       mnemonics, etc.

Mockapetris [Page 9] RFC 1101 DNS Encoding of Network Names and Other Types April 1989

    3. A "to" data type.  Since we assume several symmetrical
       mnemonic <==> number mappings, this is also necessary.
 This ordering reflects the natural scoping of control, and hence the
 order of the components in a domain name.  Thus domain names would be
 of the form:
 To make this work, we need to define well-know strings for each of
 these metavariables, as well as encoding rules for converting a
 <from-value> into a domain name.  We might define:
 <YP-origin>     :=YP
 <from-data-type>:=TCP-port | IN-ADDR | Number |
                   Assigned-network-number | Name
 <to-data-type>  :=<from-data-type>
 Note that "YP" is NOT a valid country code under [ISO 3166] (although
 we may want to worry about the future), and the existence of a
 syntactically valid <to-data-type>.<from-data-type> pair does not
 imply that a meaningful mapping exists, or is even possible.
 The encoding rules might be:
 TCP-port        Six character alphanumeric
 IN-ADDR         Reversed 4-octet decimal string
 Number          decimal integer
                 Reversed 4-octet decimal string
 Name            Domain name



 $origin Number.TCP-port.YP.
 23              PTR     TELNET.TCP-port.Number.YP.
 25              PTR     SMTP.TCP-port.Number.YP.
 $origin TCP-port.Number.YP.
 TELNET          PTR     23.Number.TCP-port.YP.

Mockapetris [Page 10] RFC 1101 DNS Encoding of Network Names and Other Types April 1989

 SMTP            PTR     25.Number.TCP-port.YP.
 Thus the mapping between 23 and TELNET is represented by a pair of
 PTR RRs, one for each direction of the mapping.

6.2. Assigned networks

 Network numbers are assigned by the NIC and reported in "Internet
 Numbers" RFCs.  To create a YP, the NIC would set up two domains:
 Name.Assigned-network-number.YP and Assigned-network-number.YP
 The first would contain entries of the form:
 $origin Name.Assigned-network-number.YP.         PTR     SATNET.Assigned-network-number.Name.YP.        PTR     ARPANET.Assigned-network-number.Name.YP.
 The second would contain entries of the form:
 $origin Assigned-network-number.Name.YP.
 SATNET.         PTR
 ARPANET.        PTR
 These YPs are not in conflict with the network name support described
 in the first half of this RFC since they map between ASSIGNED network
 names and numbers, not those allocated by the organizations
 themselves.  That is, they document the NIC's decisions about
 allocating network numbers but do not automatically track any
 renaming performed by the new owners.
 As a practical matter, we might want to create both of these domains
 to enable users on the Internet to experiment with centrally
 maintained support as well as the distributed version, or might want
 to implement only the allocated number to name mapping and request
 organizations to convert their allocated network names to the network
 names described in the distributed model.

6.3. Operational improvements

 We could imagine that all conversion routines using these YPs might
 be instructed to use "YP.<local-domain>" followed by "YP."  as a
 search list.  Thus, if the organization ISI.EDU wished to define
 locally meaningful TCP-PORT, it would define the domains:
 <TCP-port.Number.YP.ISI.EDU> and <Number.TCP-port.YP.ISI.EDU>.

Mockapetris [Page 11] RFC 1101 DNS Encoding of Network Names and Other Types April 1989

 We could add another level of indirection in the YP lookup, defining
 the <to-data-type>.<from-data-type>.<YP-origin> nodes to point to the
 YP tree, rather than being the YP tree directly.  This would enable
 entries of the form:
 IN-ADDR.Netname.YP.   PTR     IN-ADDR.ARPA.
 to splice in YPs from other origins or existing spaces.
 Another possibility would be to shorten the RDATA section of the RRs
 which map back and forth by deleting the origin.  This could be done
 either by allowing the domain name in the RDATA portion to not
 identify a real domain name, or by defining a new RR which used a
 simple text string rather than a domain name.
 Thus, we might replace
 $origin Assigned-network-number.Name.YP.
 SATNET.         PTR
 ARPANET.        PTR
 $origin Assigned-network-number.Name.YP.
 SATNET.         PTR
 ARPANET.        PTR
 $origin Assigned-network-number.Name.YP.
 SATNET.         PTT     ""
 ARPANET.        PTT     ""
 where PTT is a new type whose RDATA section is a text string.


 Drew Perkins, Mark Lottor, and Rob Austein contributed several of the
 ideas in this RFC.  Numerous contributions, criticisms, and
 compromises were produced in the IETF Domain working group and the
 NAMEDROPPERS mailing list.

Mockapetris [Page 12] RFC 1101 DNS Encoding of Network Names and Other Types April 1989


 [HR]        Braden, B., editor, "Requirements for Internet Hosts",
             RFC in preparation.
 [ISO 3166]  ISO, "Codes for the Representation of Names of
             Countries", 1981.
 [RFC 882]   Mockapetris, P., "Domain names - Concepts and
             Facilities", RFC 882, USC/Information Sciences Institute,
             November 1983.
             Superseded by RFC 1034.
 [RFC 883]   Mockapetris, P.,"Domain names - Implementation and
             Specification", RFC 883, USC/Information Sciences
             Institute, November 1983.
             Superceeded by RFC 1035.
 [RFC 920]   Postel, J. and J. Reynolds, "Domain Requirements", RFC
             920, October 1984.
             Explains the naming scheme for top level domains.
 [RFC 952]   Harrenstien, K., M. Stahl, and E. Feinler, "DoD Internet
             Host Table Specification", RFC 952, SRI, October 1985.
             Specifies the format of HOSTS.TXT, the host/address table
             replaced by the DNS
 [RFC 973]   Mockapetris, P., "Domain System Changes and
             Observations", RFC 973, USC/Information Sciences
             Institute, January 1986.
             Describes changes to RFCs 882 and 883 and reasons for
 [RFC 974]   Partridge, C., "Mail routing and the domain system", RFC
             974, CSNET CIC BBN Labs, January 1986.
             Describes the transition from HOSTS.TXT based mail
             addressing to the more powerful MX system used with the
             domain system.

Mockapetris [Page 13] RFC 1101 DNS Encoding of Network Names and Other Types April 1989

 [RFC 997]   Reynolds, J., and J. Postel, "Internet Numbers", RFC 997,
             USC/Information Sciences Institute, March 1987
             Contains network numbers, autonomous system numbers, etc.
 [RFC 1010]  Reynolds, J., and J. Postel, "Assigned Numbers", RFC
             1010, USC/Information Sciences Institute, May 1987
             Contains socket numbers and mnemonics for host names,
             operating systems, etc.
 [RFC 1034]  Mockapetris, P., "Domain names - Concepts and
             Facilities", RFC 1034, USC/Information Sciences
             Institute, November 1987.
             Introduction/overview of the DNS.
 [RFC 1035]  Mockapetris, P., "Domain names - Implementation and
             Specification", RFC 1035, USC/Information Sciences
             Institute, November 1987.
             DNS implementation instructions.

Author's Address:

 Paul Mockapetris
 USC/Information Sciences Institute
 4676 Admiralty Way
 Marina del Rey, CA 90292
 Phone: (213) 822-1511

Mockapetris [Page 14]

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