X.121

Recommendation X.121 of the International Telecommunication Union(ITU) defines the structure, characteristics and applications of the numbering plan for public data networks. This numbering plan is the prerequisite for public data networks to be able to communicate with each other worldwide.

The numbering plan specified in X.121 consists of several digits with which the public data networks in different countries and different public data networks within a country can be identified.

Structure of the International Data Number (IDN) according to X.121

X.121 describesthe structure of the numbering, namely the international identification of public data networks, of telex networks and of telecommunication networks.

As far as public data networks are concerned, there are two different International Data Number( IDN) identification schemes. One combines the Data Network Identification Code( DNIC) with the National Terminal Number( NTN), the second the Data Country Code( DCC) with the National Number ( NN).