Routing in the PSTN is the process of forwarding telephone calls between the constituent telephone networks that comprise the public switched telephone network (PSTN). Telephone calls are routed across a network of potentially many switching systems, often owned by different telephone carriers. Switching systems are connected with trunks. Each switch may have many neighbors. Neighboring switches owned by different operators are connected at interconnect points. The PSTN is a network that uses destination routing to direct calls from origin to the recipient. It is not a full mesh network with the nodes of every operator directly connected to those of every other, which would be impractical and inefficient. Therefore, calls may be routed through intermediate operator networks before they reach their final destination. Efficient least-cost routing is an important procedure in PSTN routing. Call routing Each time a call is placed for routing, the destination number (also known as the called party) is entered by the calling party into their terminal. The destination number generally has two parts, an area code which generally identifies the geographical location of the destination telephone, and a telephone number unique within that area code that determines the specific destination terminal. The telephone number may be subdivided into a prefix that may identify a more specific geographic location or a telephone exchange, and the rest of the number. For example, in the number (301) 555-1212, 301 is the area code and 555 is the prefix. (The area code is sometimes known as an "NPA," and the area-code and prefix combination is known as an "NPA-NXX.") Sometimes if the call is between two terminals in the same local area (or, both terminals are on the same telephone exchange), then the area code may be omitted, but in other places, including the area code is required for the call to complete. When a call is received by an exchange, there are two treatments that may be applied: Either the destination terminal is directly connected to that exchange, in which case the call is placed down that connection and the destination terminal rings. Or the call must be placed to one of the neighboring exchanges through a connecting trunk for onward routing. Each exchange in the chain uses pre-computed routing tables to determine which connected exchange the onward call should be routed to. There may be several alternative routes to any given destination, and the exchange can select