200-301 notes

200-301 notes.The first major part of this subsection will review IPv4 routing in the first step of this troubleshooting process: we will predict what behavior will occur under normal circumstances when IPv4 packets flow from one host to another.Thus,how does IPv4 routing work? When focusing on network-level logic,any interconnected network can be thought of as a series of routing steps. In the first step, a host creates a packet and sends it to the default router. After the packet arrives at the router, one or more routers forward (route) the packet until it reaches its destination, as shown in the first three steps of the example in Figure 4-1.The first three steps of the example in Figure 4-1 show how packets are routed through the network. Nevertheless, most applications require packets to flow in both directions. Therefore, when troubleshooting, any analysis of normal operation should take into account that packets will be sent back to the first host (in this case, PC1), as shown in steps 4, 5, and 6 in the figure.To deliver IPv4 packets end-to-end from one host to another and back requires mutual cooperation between the host and the router. This section of the book will analyze the logic based on host logic and router logic in turn.Host IPv4 Routing LogicThe IPv4 routing process begins with the host that created the IP packet. After the host creates the IP packet, the host asks the question: Is the destination IP address of this new packet in my local subnet range? The host determines the network range of the local subnet using its own IP address/mask, compares it with the range of the destination IP address, and makes the following choices.(1) If the destination is local, send it directly.a.Find the MAC address of the destination host. Use a known Address Resolution Protocol (ARP) table entry, or use an ARP message for information.b.Encapsulate the IP packet in the data link frame with the destination data link address of the destination host.-2