Network Working Group P. Mockapetris
Request for Comments: 1101 ISI
Updates: RFCs 1034, 1035 April 1989
DNS Encoding of Network Names and Other Types
1. STATUS OF THIS MEMO
This RFCproposes 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 RFCassumes that the reader is familiar with the DNS [RFC1034,
RFC1035] 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 [RFC974]), and changes described
in [RFC973]. This RFCdescribes 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,
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 [RFC952] 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
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
.
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
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 secondary
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.)
4. 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 a
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:
; 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 back
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.
5. 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 [RFC997] 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.
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 name
6. SPECIFICS FOR YP MAPPINGS
6.1. 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.
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 RFCsince 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>.
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.
8. REFERENCES
Braden, B., editor, "Requirements for Internet Hosts",
RFCin preparation.
[ISO 3166] ISO, "Codes for the Representation of Names of
Countries", 1981.
[RFC882] Mockapetris, P., "Domain names - Concepts and
Facilities", RFC882, USC/Information Sciences Institute,
November 1983.
Superseded by RFC1034.
[RFC883] Mockapetris, P.,"Domain names - Implementation and
Specification", RFC883, USC/Information Sciences
Institute, November 1983.
Superceeded by RFC1035.
[RFC920] Postel, J. and J. Reynolds, "Domain Requirements", RFC
920, October 1984.
Explains the naming scheme for top level domains.
[RFC952] Harrenstien, K., M. Stahl, and E. Feinler, "DoD Internet
Host Table Specification", RFC952, SRI, October 1985.
Specifies the format of HOSTS.TXT, the host/address table
replaced by the DNS
[RFC973] Mockapetris, P., "Domain System Changes and
Observations", RFC973, USC/Information Sciences
Institute, January 1986.
Describes changes to RFCs 882 and 883 and reasons for
them.
[RFC974] 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.
[RFC997] Reynolds, J., and J. Postel, "Internet Numbers", RFC997,
USC/Information Sciences Institute, March 1987
Contains network numbers, autonomous system numbers, etc.
[RFC1010] 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.
[RFC1034] Mockapetris, P., "Domain names - Concepts and
Facilities", RFC1034, USC/Information Sciences
Institute, November 1987.
Introduction/overview of the DNS.
[RFC1035] Mockapetris, P., "Domain names - Implementation and
Specification", RFC1035, 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.EDU
