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Router Security Configuration Guide UNCLASSIFIED 16 UNCLASSIFIED Version 1.0g network data messages from a LAN and convert them into packets suitable for transmission beyond the LAN on a wide area network (WAN).  The goal is almost always to get these packets to another LAN and ultimately to the correct host on that LAN. Part of the “conversion” process is to add a packet header. Other routers will generally only look at a packet’s header information, not at the contents or data in the packet.   Routers also make decisions about where to send these packets, based on: the addresses contained within the packet headers and a table of routes maintained within the router.  Updating these routing tables and forwarding data packets between portions of a network are one of the primary purposes of a router.  Building packets and unwrapping packets are additional router functions performed by the first and last routers, respectively, that a message passes through.  In addition to directing packets, a router may be responsible for filtering traffic, allowing some packets to pass through and rejecting others.  Filtering can be a very important function of routers; it allows them to help protect computers and other network components.  For more information about filtering, see Section 3 and Section 4. It is also possible that at the destination end a router may have to break large packets up to accommodate the size limits of the destination LAN. There is no reason that routers cannot be used to send messages between hosts (as shown in Figure 1-1) but more typically routers are used to connect LANs to each other or to connect a LAN to a WAN.    Most large computer networks use the TCP/IP protocol suite.  In some sense this is the lingua franca of the Internet.  See Section 2.2 for a quick review of TCP/IP and IP addressing.    2.1.2.    Routing Tables As mentioned, one of tasks of a router is to maintain routing tables which are used to decide where a packet is to go and thus which interface it should be sent out. In the past these tables were built and updated by hand and this is referred to as static routing. In dynamic routing, the router learns about where various addresses are relative to itself and builds up routing tables based on this information. There are a number of schemes or routing protocols for routers to acquire and share routing table information. While a thorough treatment of the details is beyond the scope of this document, there is a brief discussion of routing protocols is in Section 4.4.