X.121 addresses are similar to long-distance telephone numbers and are used by X.25 end nodes to call each other to set up communication sessions. X.121 addresses are used during the call setup phase of X.25 communication and are used to establish a virtual circuit between the source node and destination node on the network.
X.121 addresses can be up to 14 decimal digits in length (if that many digits are required to uniquely determine the address of the destination node being called). The first four digits form the data network identification code (DNIC), with the first three digits indicating the country and the fourth digit indicating the carrier that owns the common packet-switching network being used to make the call. The last 8 to10 digits form the national terminal number (NTN) and identify the end node being called. An additional 1-byte header indicates the number of digits of both the source and destination nodes.
Once a communication session is established, a 12-bit logical channel identifier (LCI) is assigned to the two hosts as the identification number of the virtual circuit that is established between them. The X.25 network uses the LCI in the headers of the X.25 packets for routing data between the nodes. The X.121 address is used only at call setup to establish the virtual circuit.