Appendix A. Address Formats

X.121 Addresses

X.25 addresses are X.121 DTE strings that contain a maximum of four fields. The format of the address depends on the type of network and the version of X.25 recommendation that the network uses. Table A-1 illustrates X.121 address formats.

Table A-1. X.25 Network Address Formats

Network Type

X.25 Version

Address Formats


1984 and 1988



id.LSAP.X.non-OSI address extension




id.LCI.P (for permanent virtual circuits)


1984 and 1988


id.DTE.X.non-OSI address extension

id.LCI.P (for permanent virtual circuits)

The fields in the previous address formats have these meanings:


is a single-character (alphanumeric) subnetwork identifier. This identifies a connection to or from the WAN or LAN.


defines the Subnetwork Point of Attachment (SNPA) for each WAN line. This is entered as an address of up to 15 decimal digits; or, for X.25(88) TOA/NPI addresses, a maximum of 17 decimal digits.


defines the SNPA for each LAN line. It comprises the MAC address plus the DSAP address, 14 hexadecimal digits in all:

The MAC address is a unique 12-digit address identifying hosts and devices on the Ethernet; it is also called an Ethernet address. An MAC address looks like this example:


The DSAP address is the Destination Service Access Point, which is a two-digit address indicating the destination Link Layer service access point.

N or X 

is an indicator that identifies the next part of the address as either an OSI NSAP address or a non-OSI address extension.


is the Network Service Access Point, the point at which the Network Service is provided. The NSAP address can be up to 40 hexadecimal digits.

non-OSI address extension  

indicates that the given address (up to 40 hexadecimal digits) is encoded in the X.25 Call Request packet as an “eXtended” address, that is, non-OSI, as opposed to an NSAP address (OSI).


is the logical channel identifier, three hexadecimal digits. Pad in front with zeroes if necessary, for example, enter “009” rather than “9”. (The “P” that follows indicates that the LCI describes a permanent virtual circuit.)

Internet Addresses

An Internet address has 32 bits. It can be thought of as containing four fields, each ranging from 0 to 255 (decimal). The four fields are usually represented with “.” field delimiters. An Internet Address can be represented as decimal, octal, or hexadecimal:

  • Decimal addresses have no prefixes and may not begin with zero. An example is “”.

  • Octal addresses begin with a zero. For example, “” in decimal is “0123.0.0.177” in octal.

  • Hexadecimal addresses are preceded by the 0x or 0X prefix. For example, “” in decimal is “0x53.0x0x0x7F”.