from external connections but is not recognized as a security feature by all experts. Some experts argue that open source routers are more secure and reliable than closed source routers because open-source routers allow mistakes to be quickly found and corrected. Routing different networks Routers are also often distinguished on the basis of the network in which they operate. A router in a local area network (LAN) of a single organisation is called an interior router. A router that is operated in the Internet backbone is described as exterior router. While a router that connects a LAN with the Internet or a wide area network (WAN) is called a border router, or gateway router. Internet connectivity and internal use Routers intended for ISP and major enterprise connectivity usually exchange routing information using the Border Gateway Protocol (BGP). defines the types of BGP routers according to their functions: Edge router (also called a provider edge router): Placed at the edge of an ISP network. The router uses Exterior Border Gateway Protocol (EBGP) to routers at other ISPs or large enterprise autonomous systems. Subscriber edge router (also called a customer edge router): Located at the edge of the subscriber's network, it also uses EBGP to its provider's autonomous system. It is typically used in an (enterprise) organization. Inter-provider border router: A BGP router for interconnecting ISPs that maintains BGP sessions with other BGP routers in ISP Autonomous Systems. Core router: Resides within an Autonomous System as a backbone to carry traffic between edge routers. Within an ISP: In the ISP's autonomous system, a router uses internal BGP to communicate with other ISP edge routers, other intranet core routers, or the ISP's intranet provider border routers. Internet backbone: The Internet no longer has a clearly identifiable backbone, unlike its predecessor networks. See default-free zone (DFZ). The major ISPs' system routers make up what could be considered to be the current Internet backbone core. ISPs operate all four types of the BGP routers described here. An ISP core router is used to interconnect its edge and border routers. Core routers may also have specialized functions in virtual private networks based on a combination of BGP and Multi-Protocol Label Switching protocols. Port forwarding: Routers are also used for port forwarding between private Internet-connected servers. Voice, data, fax, and video processing routers: Commonly referred to as access servers or gateways, these devices are used to route