CCNA Cisco Certified Network Associate Study Guide - FTP Server

238 Chapter 5 � IP Routing
In our example, a user on Host A pings Host B’s IP address. It will not get
simpler than this. Let’s work through the steps.
1. From a command prompt, the user types ping 172.16.20.2. A packet
is generated on the Host A machine using the IP and ICMP Network
layer protocols.
2. IP works with the ARP protocol to determine what network this packet
is destined for by looking at the IP address and the subnet mask of Host
A. Since this is a request for a remote host, which means it is not destined
to be sent to a host on the local network, the packet must be sent
to the router so that it will be routed to the correct remote network.
3. For Host A to send the packet to the router, it must know the hardware
address of the router’s interface located on the local network.
Remember that the Network layer will hand the packet and the destination
hardware address to the Data Link layer for framing and transmitting
on a local host. To get the hardware address, the host looks in
a location in memory called the ARP cache.
4. If the IP address has not already been resolved to a hardware address
and is not in the ARP cache, the host sends an ARP broadcast looking,
for the hardware address of IP address 172.16.10.1. This is why the
first Ping usually times out, and the other four are successful. After the
address is cached, no timeouts usually occur.
5. The router responds with the hardware address of the Ethernet interface
connected to the local network. The host now has everything it
needs to transmit the packet out on the local network to the router.
The Network layer hands down the packet it generated with the ICMP
echo request (Ping) to the Data Link layer, along with the hardware
address of where the host wants to send the packet. The packet
includes the IP source address and the destination IP address, as well
as the ICMP specified in the Network layer protocol field.
6. The Data Link layer creates a frame, which encapsulates the packet
with the control information needed to transmit on the local network.
This includes the source and destination hardware addresses and the
type field specifying the Network layer protocol (it is a type field since
IP uses an Ethernet_II frame by default). Figure 5.2 shows the frame
that will be generated by the Data Link layer and sent out on the local
media.