he Classless Interdomain Routing (CIDR) prefix for newer protocols. (CIDR is discussed later in this chapter.) OSPF OSPF, which is sanctioned by the IETF and supported by TCP, is per- haps the most widely used interior routing protocol in large networks. OSPF is a link-state protocol that has a complex set of options and features. It makes use of Dijkstra's algorithm, which determines routes based on path length, calculates the shortest-path tree, and uses cost as its routing metric. An OSPF network is divided into areas, or logical groupings of routers whose information can be summarized and sent toward the rest of the network. The core of the network is formed by a special area called the backbone area, and all interarea routing occurs via this backbone. All areas must connect to the backbone, and if no direct connection is possible, a virtual link may be established. The other areas, which vary in their ability to receive external and/or summary route information, are as follows: Stub area (SA)- - SA does not receive any external routes and needs to rely on a default route to send traffic to routes outside the immediate domain. (A default route is the network route used by a router or server when no other known route works for a given IP packet's destination address.) Hosts and routers in an organization generally point the default route toward the router that has a connection to a network service provider to ensure that packets destined for the Internet will be sent toward the router that has the Internet connection. Totally stubby area (TSA)-The TSA does not receive any external routes or summary routes. The only way for traffic to get routed outside the area is by using a default route that is the only link advertised into the area. Not-so-stubby area (NSSA)-The NSSA can import autonomous system external routes and send them to the backbone but cannot receive autono- mous system external routes from the backbone or other areas. Internet Basics OSPF defines various router types, which are also logical defini