 on the network addresses, distance, cost, and availability. Adapting to network conditions-A router adapts to network conditions by changing what it considers to be the best or optimum paths, depending on the network traffic status. How do routers actually work? A router has input ports for receiving packets and output ports for sending those packets toward their destination. When the packet comes to the input port, the router examines the packet header and checks the destination against a routing table (i.e., a database that tells the router how to send packets to various destinations). Based on the information in the routing table, the router sends the packet to a particular output port, and the output port sends the packets-possibly to the destination, or possibly to another router that is one step closer to that packet's destination. Packets are delivered from node to node (i.e., router to router) because, at each node, the router modifies the MAC address to be that of the next node. The destination network address does not change, of course, but the destination MAC changes at each node; this is the only way the packet can travel from one node to the next. If packets come to the input port more quickly than the router can process them, they are sent to a holding area called an input queue. The router then pro- cesses packets from the queue in the order in which they were received. If the number of packets received exceeds the length of the queue, packets may be lost due to the router running out of available memory. When this happens, an error control mechanism (such as TCP) that is housed on the sending and receiving computers has the packets resent. (You'll learn more about TCP in Chapter 8, "The Internet and IP Infrastructures.") Types of Routing Tables There are two types of routing tables: Static-The simpler kind of routing table is the static routing table. In a static routing table, there are specific ways of routing data to other net- works, and only those paths can be used. New routes can be added t