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Network Working Group                                     P. MockapetrisRequest for Comments: 1101                                           ISIUpdates: RFCs1034,1035                                      April 1989DNS Encoding of Network Names and Other Types1. STATUS OF THIS MEMO   This RFC proposes two extensions to the Domain Name System:      - A specific method for entering and retrieving RRs which map        between network names and numbers.      - 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.2. INTRODUCTION   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:      - Several tradeoffs involving control of network names, the        syntax of network names, backward compatibility, etc.      - 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   recommendations.   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).3. NETWORK NAME ISSUES AND DISCUSSION   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 ofMockapetris                                                     [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 26.0.0.73.COM 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,   are:      - 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.      - Mapping a network name to a network address.        This facility is of less obvious application, but a        symmetrical mapping seems desirable.      - 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.      - 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 secondaryMockapetris                                                     [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:      - 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.      - 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, 0.0.0.10.IN-ADDR.ARPA for class A net 10,        0.0.2.128.IN-ADDR.arpa 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        database.      - 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        boundaries.)Mockapetris                                                     [Page 4]

RFC 1101     DNS Encoding of Network Names and Other Types    April 19894. SPECIFICS FOR NETWORK NAME MAPPINGS   The proposed solution uses information stored at:      - 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, 0.0.0.10.IN-ADDR.ARPA. 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:        0.0.0.10.IN-ADDR.ARPA.  PTR     ARPANET.ARPA.        or        0.0.2.128.IN-ADDR.ARPA. PTR     cmu-net.cmu.edu.                                A       255.255.255.0        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        entries.      - 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     0.0.0.10.IN-ADDR.ARPA.        or        isi-net.isi.edu.        PTR     0.0.9.128.IN-ADDR.ARPA.        In general, this information will be inserted in the master        file for the domain name of the organization; this is aMockapetris                                                     [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.      - 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        networks.        For example:        ISI.EDU.        PTR     0.0.9.128.IN-ADDR.ARPA.        MCC.COM.        PTR     0.167.5.192.IN-ADDR.ARPA.                        PTR     0.168.5.192.IN-ADDR.ARPA.                        PTR     0.169.5.192.IN-ADDR.ARPA.                        PTR     0.0.62.128.IN-ADDR.ARPA.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     0.0.0.10.IN-ADDR.ARPA.   ARPANET.ARPA.           PTR     0.0.0.10.IN-ADDR.ARPA.   0.0.0.10.IN-ADDR.ARPA.  PTR     ARPANET.ARPA.   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   named ARPANET.ARPA.   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   isi-net.isi.edu.                PTR  0.0.9.128.IN-ADDR.ARPA.   ; Define first level subnets   div1-subnet.isi.edu.            PTR  0.1.9.128.IN-ADDR.ARPA.   div2-subnet.isi.edu.            PTR  0.2.9.128.IN-ADDR.ARPA.   ; Define second level subnets   inc-subsubnet.isi.edu.          PTR  16.2.9.128.IN-ADDR.ARPA.   in the 9.128.IN-ADDR.ARPA zone:   ; Define network number and address mask   0.0.9.128.IN-ADDR.ARPA.         PTR  isi-net.isi.edu.                                   A    255.255.255.0  ;aka X'FFFFFF00'   ; Define one of the first level subnet numbers and masks   0.1.9.128.IN-ADDR.ARPA.         PTR  div1-subnet.isi.edu.                                   A    255.255.255.240 ;aka X'FFFFFFF0'   ; Define another first level subnet number and mask   0.2.9.128.IN-ADDR.ARPA.         PTR  div2-subnet.isi.edu.                                   A    255.255.255.240 ;aka X'FFFFFFF0'   ; Define second level subnet number   16.2.9.128.IN-ADDR.ARPA.        PTR  inc-subsubnet.isi.edu.   This assumes that the ISI network is named isi-net.isi.edu., first   level subnets are named div1-subnet.isi.edu. and div2-   subnet.isi.edu., and a second level subnet is called inc-   subsubnet.isi.edu.  (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 10.0.0.51.      1. Since this is a class A address, use a mask x'FF000000' and         get 10.0.0.0.      2. Construct the name 0.0.0.10.IN-ADDR.ARPA.      3. Do a PTR query.  Get backMockapetris                                                     [Page 7]

RFC 1101     DNS Encoding of Network Names and Other Types    April 1989         0.0.0.10.IN-ADDR.ARPA.  PTR     ARPANET.ARPA.      4. Conclude that the network name is "ARPANET.ARPA."   Suppose that the IP address is 128.9.2.17.      1. Since this is a class B address, use a mask of x'FFFF0000'         and get 128.9.0.0.      2. Construct the name 0.0.9.128.IN-ADDR.ARPA.      3. Do a PTR query.  Get back         0.0.9.128.IN-ADDR.ARPA.       PTR     isi-net.isi.edu      4. Conclude that the network name is "isi-net.isi.edu."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 128.9.2.17.      1. As before construct a query for 0.0.9.128.IN-ADDR.ARPA.         Retrieve:         0.0.9.128.IN-ADDR.ARPA.  PTR    isi-net.isi.edu.                                  A      255.255.255.0      2. Since an A RR was found, repeat using mask from RR         (255.255.255.0), constructing a query for         0.2.9.128.IN-ADDR.ARPA.  Retrieve:         0.2.9.128.IN-ADDR.ARPA.  PTR    div2-subnet.isi.edu.                                  A      255.255.255.240      3. Since another A RR was found, repeat using mask         255.255.255.240 (x'FFFFFFF0').  constructing a query for         16.2.9.128.IN-ADDR.ARPA.  Retrieve:         16.2.9.128.IN-ADDR.ARPA. PTR    inc-subsubnet.isi.edu.      4. Since no A RR is present at 16.2.9.128.IN-ADDR.ARPA., there         are no more subnet levels.Mockapetris                                                     [Page 8]

RFC 1101     DNS Encoding of Network Names and Other Types    April 19895. YP ISSUES AND DISCUSSION   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:   <from-value>.<to-data-type>.<from-data-type>.<YP-origin>   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   Assigned-network-number                   Reversed 4-octet decimal string   Name            Domain name6. SPECIFICS FOR YP MAPPINGS6.1. TCP-PORT   $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.   0.0.0.4         PTR     SATNET.Assigned-network-number.Name.YP.   0.0.0.10        PTR     ARPANET.Assigned-network-number.Name.YP.   The second would contain entries of the form:   $origin Assigned-network-number.Name.YP.   SATNET.         PTR     0.0.0.4.Name.Assigned-network-number.YP.   ARPANET.        PTR     0.0.0.10.Name.Assigned-network-number.YP.   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     0.0.0.4.Name.Assigned-network-number.YP.   ARPANET.        PTR     0.0.0.10.Name.Assigned-network-number.YP.   with   $origin Assigned-network-number.Name.YP.   SATNET.         PTR     0.0.0.4.   ARPANET.        PTR     0.0.0.10.   or   $origin Assigned-network-number.Name.YP.   SATNET.         PTT     "0.0.0.4"   ARPANET.        PTT     "0.0.0.10"   where PTT is a new type whose RDATA section is a text string.7. ACKNOWLEDGMENTS   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 19898. REFERENCES   [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 byRFC 1034.   [RFC 883]   Mockapetris, P.,"Domain names - Implementation and               Specification",RFC 883, USC/Information Sciences               Institute, November 1983.               Superceeded byRFC 1035.   [RFC 920]   Postel, J. and J. Reynolds, "Domain Requirements",RFC920, 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               them.   [RFC 974]   Partridge, C., "Mail routing and the domain system",RFC974, 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",RFC1010, 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   Email: PVM@ISI.EDUMockapetris                                                    [Page 14]