IP to X.121 address mapping for DDN (RFC1236)
Original Publication Date: 1991-Jun-01
Included in the Prior Art Database: 2019-Feb-11
Internet Society Requests For Comment (RFCs)
L. Morales: AUTHOR [+1]
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.
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.
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" . The translation method described below is TOTALLY independent of IP subnetting and of any masking that may be used in support of IP subnetting.
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  (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 addres...