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rfc:rfc1236

Network Working Group L. Morales Request for Comments: 1236 P. Hasse

                                                               USAISEC
                                                             June 1991
                IP to X.121 Address Mapping for DDN

Status of this Memo

 This memo defines a standard way of converting IP addresses to CCITT
 X.121 addresses and is the recommended standard for use on the
 Internet, specifically for the Defense Data Network (DDN).  This memo
 provides information for the Internet community.  It does not specify
 an Internet standard.  Distribution of this memo is unlimited.

1. Overview

 The Defense Communication Agency (DCA) has stated that "DDN specifies
 a standard for mapping Class A addresses to X.121 addresses."
 Additionally DCA has stated that Class B and C IP to X.121 address
 mapping "standards are the responsibility of the administration of
 the Class B or C network in question".  Therefore, there is NO
 defined single standard way of converting Class B and Class C IP
 addresses to X.121 addresses.
 This is an important issue because currently there is no way for
 administrators to define IP to X.121 address mapping.  Without a
 single standard, in a multi-vendor network environment, there is no
 assurance that devices using IP and DDN X.25 will communicate with
 each other.
 The IP to X.121 address mapping of Class B and Class C IP addresses
 shall be implemented as described below.  This translation method is
 a direct expansion of the algorithm described in the "MIL-STD:  X.25,
 DDN X.25 Host Interface Specification" [1].  The translation method
 described below is TOTALLY independent of IP subnetting and of any
 masking that may be used in support of IP subnetting.

2. Background

 All Internet hosts are assigned a four octet (32 bit) address
 composed of a network field and a local address field also known as
 the REST field [2] (see Figure 1 thru 3).  Two basic forms of
 addresses are provided:  (1) Physical addresses, correspond to the
 node number and DCE port number of the node to which the DTE is
 connected.  (2) Logical addresses, are mapped transparently by DCE
 software into a corresponding physical network address.

Morales & Hasse [Page 1] RFC 1236 IP to X.121 Address Mapping for DDN June 1991

 To provide flexibility, Internet addresses are divided into 3 primary
 classes:  Class A, Class B, and Class C.  These classes allow for a
 large number of small and medium sized networks.  The network
 addresses used within the Internet in Class A, B, and C networks are
 divided between Research, Defense, Government, (Non-Defense) and
 Commercial uses.
 As described in the MIL-STD:  X25, an IP address consists of the
 ASCII text string representation of four decimal numbers separated by
 periods, corresponding to the four octets of a thirty-two bit
 Internet address.  The four decimal numbers are referred to in this
 memo as network (n), host (h), logical address (l), and Interface
 Message Processor (IMP) or Packet Switch Node (PSN) (i).  Thus, an
 Internet address maybe represented as "n.h.l.i" (Class A), "n.n.h.i"
 (Class B), or "n.n.n.hi" (Class C), depending on the Internet address
 class.  Each of these four numbers will have either one, two, or
 three decimal digits and will never have a value greater than 255.
 For example, in the Class A IP address "26.9.0.122", n=26 h=9, l=0,
 and i=122.
 The different classes of Internet addresses [3] are illustrated
 below:
 Class A:
 The highest-order bit is set to 0.
 7-bits define the network number.
 24-bits define the local address.
 This allows  up to 126 class A networks.
 Networks 0 and 127 are reserved.
    |       n       |       h       |       l       |       i       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|   NETWORK   |                 Local Address                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           7 Bits                  24 Bits (REST Field)
                                 Figure 1
 Class B:
 The two highest-order bits are set to 1-0.
 14-bits define the network number.
 16-bits define the local address.
 This allows up to 16,384 class B networks.

Morales & Hasse [Page 2] RFC 1236 IP to X.121 Address Mapping for DDN June 1991

    |        n      |       n       |       h       |       i       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1 0|           NETWORK         |          Local Address        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    14 Bits                 16 Bits (REST Field)
                                 Figure 2
 Class C:
 The three highest-order bits are set to 1-1-0.
 21-bits define the network number.
 8-bits define the local address.
 This allows up to 2,097,152 class C networks
    |       n       |       n       |       n       |   h   |   i   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1 1 0|                 NETWORK                 | Local Address |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                            21 Bits                8 Bits (REST Field)
                                 Figure 3
 The fourth type of address, class D, is used as a multicast address.
 The four highest-order bits are set to 1-1-1-0.  Note:  No addresses
 are allowed with the four highest-order bits set to 1-1-1-1.  These
 addresses, called "class E", are reserved.
 The "MIL-STD:  X.25" states "All DDN addresses are either twelve or
 fourteen BCD (binary-coded decimal) digits in length.".  The last two
 digits are referred to as the Sub-Address and are not used on the
 DDN.  The Sub-Address is carried across the network without
 modification.  Its presence is optional.  Therefore, a DTE may
 generate EITHER a twelve or fourteen BCD X.121 address, but must
 accept both twelve and fourteen BCD X.121 addresses.

3. Standard IP to X.121 Address Mapping

 This section describes the algorithm that should be used to convert
 IP addresses to X.121 addresses [1].  You will note that "h" is
 always listed as greater than or less than the number 64.  This
 number is used to differentiate between PSN physical and logical host
 port addresses.  Note that at the time of this writing, the DDN does
 not make use of the PSN's logical addressing feature, which allows
 hosts to be addressed independently of their physical point of
 attachment to the network.

Morales & Hasse [Page 3] RFC 1236 IP to X.121 Address Mapping for DDN June 1991

3.1 Derivation of DDN X.25 Addresses

 To convert a Class A IP address to a DDN X.25 address:

3.1.1 If the host field (h) is less than 64 (h < 64),

     the address corresponds to the following DDN X.25
     physical address:
 ZZZZ F III HH ZZ (SS)
 where:
 ZZZZ = 0000
 F = 0   because the address is a physical address;
 III     is a three decimal digit representation of "i",
         right-adjusted and padded with leading zeros if required;
 HH      is a two decimal digit representation of h", right-adjusted
         and padded with leading zeros if required;
 ZZ = 00 is optional.
 (SS)    is an optional Sub-Address field which is ignored in the DDN.
         This field is either left out or filled with zeros.
 The address 26.9.0.122 corresponds to the DDN X.25 physical address
 000001220900.

3.1.2. If the host field (h) is greater than or equal to

      64 (h >= 64), the address corresponds to the following
      DDN X.25 physical address:
 ZZZZ F RRRRR ZZ (SS)
 where:
 ZZZZ = 0000
 F = 1   because the address is a logical address;
 RRRRR   is a five decimal digit representation of the result "r" of
         the calculation
 r = h * 256 + i
 (note that the decimal representation of "r" will always require five

Morales & Hasse [Page 4] RFC 1236 IP to X.121 Address Mapping for DDN June 1991

 digits)
 ZZ = 00
 and
 (SS)   is optional.
 The address 26.83.0.207 corresponds to the DDN X.25 logical address
 000012145500.

3.2. For Class B IP addresses the "h" and "i" fields will ALWAYS

    consist of 8 bits each taken from the REST field of the Internet
    address.  The mapping follows the same rules as in 3.1.

3.3. For Class C IP addresses the "h" and "i" fields will ALWAYS

    consist of 4 bits each taken from the REST field of the Internet
    address.  The mapping follows the same rules as in 3.1.

4. Examples

 The following are examples of IP to X.121 address mappings for Class
 A, Class B, and Class C IP addresses.

4.1 Class A

 The mapping of X.121 address for Class A networks:
        for h < 64
        example: 26.29.0.122   format: n.h.l.i
                 ZZZZ F III HH ZZ (SS)
 X.121 address = 0000 0 122 29 00  00
        for h > or = 64
        example: 26.80.0.122   format: n.h.l.i
                  ZZZZ F RRRRR ZZ (SS)
 X.121 address =  0000 1 20602 00  00
                where R = H * 256 + I

4.2 Class B

 The mapping of X.121 address for Class B networks:

Morales & Hasse [Page 5] RFC 1236 IP to X.121 Address Mapping for DDN June 1991

        for h < 64
        example: 137.80.1.5     format: n.n.h.i
                  ZZZZ F III HH ZZ (SS)
 X.121 address =  0000 0 005 01 00  00
        for h > or = 64
        example: 137.80.75.2    format: n.n.h.i
                  ZZZZ 1 RRRRR ZZ  (SS)
 X.121 address =  0000 1 19202 00  00
                where R = H * 256 + I

4.3 Class C

 The mapping of X.121 address for Class C networks:
        for h < 64
         example: 192.33.50.19  format: n.n.n.hi
                           H    I
                    n.n.n.0001 0011
                           1    3
            Subnet  1
            Subhost 3
                  ZZZZ F III HH ZZ (SS)
 X.121 address =  0000 0 003 01 00  00
 NOTE:  The mapping of X.121 address for Class C networks for h > 64
 is not applicable since the "h" field can never exceed 15.

5. References

 [1] MIL-STD:  X.25 "Defense Data Network X.25 Host Interface
     Specification", Defence Communications Agency, BBN Communications
     Corporation, 1983 December, Volume 1 of the "DDN Protocol
     Handbook" (NIC 50004).  Also available online at the DDN NIC as
     NETINFO:X.25.DOC.
 [2] MIL-STD:  1777 "Internet Protocol", 1983 August, Volume 1 of the
     "DDN Protocol Handbook" (NIC 50004).

Morales & Hasse [Page 6] RFC 1236 IP to X.121 Address Mapping for DDN June 1991

 [3] Kirkpatrick, S., M. Stahl, and M. Recker, "Internet Numbers", RFC
     1166, DDN NIC, July 1990.
     (Unless otherwise indicated, copies of federal and military
     specifications, standards, and handbooks are available from the
     Naval Publications and Forms Center, (ATTN:  NPODS), 5801 Tabor
     Avenue, Philadelphia, PA 19120-5099.)

6. Security Considerations

 Security issues are not discussed in this memo.

7. Authors' Addresses

 Luis F. Morales, Jr.
 USAISEC
 ASQB-SEP-C
 Ft. Huachuca, AZ 85613-5300
 Phone:  (602) 533-2873
 EMail:  lmorales@huachuca-emh8.army.mil
 Phillip R. Hasse
 USAISEC
 ASQB-SEP-C
 Ft. Huachuca, AZ 85613-5300
 Phone:  (602) 533-2873
 EMail:  phasse@huachuca-emh8.army.mil

Morales & Hasse [Page 7]

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