The objective of numbering plan is to uniquely identify every subscriber connected to a telecommunication network. A numbering plan may be open, semiopen or closed. An open-numbering plan permits wide variation in the number of digits to be used to identify a subscriber within a multi exchange area or within a country. This plan is used in countries equipped extensively with non-Director Strowger switching systems. In such cases the numbering scheme is usually an exact image of the network structure changes. A semi-open plan permits number lengths to differ by almost one or two digits. Today, this scheme is the most common and is used in many countries including India. In closed numbering plan or the Uniform numbering scheme, the number of digits in a subscriber number is fixed. An international numbering plan or world numbering plan has been defined by CCITT in its recommendations E.160-E.163. For numbering purposes, the world is divided into zones as shown in Fig. Each zone is given a single digit code. For the European zone two codes have been allotted because of the large number of countries within this zone. Every international telephone number consists of two parts as shown in Fig. The country code contains one, two or three digits, the first digit being the zone code in which the country lies.. In cases where an integrated numbering plan already covers an entire zone, the countries in that zone are identified by the single digit zone code itself. The existence of world numbering plan places restriction on the national numbering plan of each country. The number of digits in an international subscriber number is limited to an absolute maximum of 12. In practical, with a few exceptions, world numbers are limited to 11 digits. As a result, the number of digits available for a national numbering plan is 11-N, where N is the number of digits in the country code. In general, a national number consist of three parts as shown in Fig. The area or the trunk code identify a particular numbering area or the multi exchange area of the called subscriber, and thus determine the routing for a trunk call and a charge for it. According to CCITT international usage, a numbering area is identified as that area in which any two subscriber use identical dialing procedure to reach any other subscriber in the network. An exchange code identifies a particular exchange within a numbering area. It determine the routing for incoming trunk call from another numbering area or for a call originating from one exchange and destined to another in the same numbering area. Subscriber line number is used to select the called subscriber line at the terminating exchange. In CCITT terminology, the combination of the exchange code and the subscriber line number is known as the subscriber number which is the number listed in the telephone directory.