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Router Security Configuration Guide UNCLASSIFIED 84 UNCLASSIFIED Version 1.0g 4.4.  Routing and Routing Protocols  “A protocol is a formal description of a set of rules and conventions that govern how devices on a network exchange information.”[5] This section will discuss two basic types of protocols, with a focus on the latter. The two types of protocols are: § Routed protocols – These are protocols that can be routed by a router. The routed protocol allows the router to correctly interpret the logical network. Some examples of routed protocols are IP, IPX, AppleTalk, and DECnet. § Routing protocols –   “A routing protocol gathers information about available networks and the distance, or cost, to reach those networks.”[7] These protocols support routed protocols and are used to maintain routing tables. Some examples of routing protocols are OSPF, RIP, BGP, and IGRP. All of the examples in this section are based on the sample network architecture shown in Figure 4-1.   Routed Protocols The most commonly used routed network protocol suite is the TCP/IP suite; its foundation is the Internet Protocol (IP).  This section will not provide an in depth discussion of this protocol, as that is far beyond the scope of this document, consult [6] for a detailed introduction. ARPA developed IP over twenty-five years ago under the ARPANET project. Today, it has grown in popularity and is the most widely implemented standard in use today.  Its growth and popularity can be attributed to IP’s ability to connect different networks regardless of different physical environments, and the flexibility and open nature of the IP network architecture. IP is designed for use on large networks; using IP, a connected host anywhere on a network can communicate with any other. (In practice, software applications running on hosts almost never use raw IP to communicate.  Instead, they use one of two transport-layer protocols built on top of IP: the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP).  Whether applications use TCP or UDP is immaterial to routing, which takes place exclusively at the IP layer.)  Further, each IP host does not need to know a path through the network to every other host.  Each host only needs to know the address of one or a small number of routers.  These routers are responsible for ensuring that each IP packet reaches its intended destination.    In a small network, each router can simply be connected directly to every other router.  For larger networks, of course, connecting every router to every other would be prohibitively expensive.  Instead, each router maintains a route table with information about how to forward packets to their destination addresses.  Correct, efficient, and secure operation of any large IP network depends on the integrity of its route tables.