Introduction to Tandem X.25 Capabilities
Addressing in X.25 Networks
What is X.25?
1–10 065307 Tandem Computers Incorporated
Addressing in X.25
Networks
Every subscriber to a network must have a unique address. An international
numbering plan, specified by CCITT standard X.121, dictates the format of each
address. This standard originally applied to public data networks, but is now used
also for connections to private networks.
Every connection is allocated an address which can contain up to 15 digits (Figure 1-7).
Figure 1-7. X.121 Address Format
007
P + DNIC NTN
+
Prefix Data Network
Identification Code
Network Terminal
Number
4 digits 10 digits
maximum
1 digit
An X.121 address uniquely identifies a country or geographical area, a network, and a
subscriber, as follows:
P The first digit is a prefix. It is used for international calls from a public data
network. It identifies a call being made to a subscriber on a public data
network in a different country. This digit is either 0 (zero) or 1 (one),
depending on the network where the call originates.
DNIC The next four digits are the Data Network Identification Code (DNIC), which
consists of a three-digit country code (Data Country Code), followed by a
one-digit network identifier.
A large country with many networks, such as the United States, might have
more than one country code assigned to it. For example, the United States
has country codes 310 through 319. The DNIC for TYMNET is 3106 and for
SPRINT is 3110.
NTN The last ten digits make up the Network Terminal Number (NTN). The NTN
identifies the DTE that is connected to the X.25 network.
Port Numbers or
Subaddressing
Depending on the network addressing scheme, the port number or subaddress can
either be part of the called address or part of the call user data. If the port number is
part of the called address, some portion of the NTN may be used An additional digit
or digits may also be added for subaddressing. Subaddressing directs calls to specific
devices or applications by assigning different port numbers for each device (terminal
or printer) or application on an X.25 link.
(Subaddressing is similar to the way a company telephone system might work: an
outside caller can either reach the company receptionist by dialing a main number or