CCNA Cisco Certified Network Associate Study Guide - FTP Server

The IP Routing Process 241
The frame generated from the router’s Ethernet 1 interface has the
source hardware address of the Ethernet 1 interface and the hardware
destination address of Host B’s network interface card. However, the
most important thing here is that even though the frame’s source and
destination hardware address changed at every interface of the router
it was sent to and from, the IP source and destination addresses never
changed. The packet was never modified at all; only the frame changed.
13. Host B receives the frame and runs a CRC. If that checks out, it discards
the frame and hands the packet to IP. IP will then check the destination
IP address. Since the IP destination address matches the IP
configuration of Host B, it looks in the protocol field of the packet to
determine what the purpose of the packet is.
14. Since the packet is an ICMP echo request, Host B generates a new
ICMP echo-reply packet with a source IP address of Host B and a destination
IP address of Host A. The process starts all over again, except
that it goes in the opposite direction. However, the hardware address
of each device along the path is already known, so each device only
needs to look in its ARP cache to determine the hardware address of
each interface.
If you had a much larger network, the process would be the same, with the
packet simply going through more hops before it finds the destination host.
IP Routing in a Larger Network
In the example given in the previous section, the routing table of the router
already has both IP networks in the routing table because the network is
directly connected to the router. But what if we add three more routers? Figure
5.4 shows four routers, 2500A, 2500B, 2500C, and 2621A. These routers,
by default, only know about their directly connected networks.
FIGURE 5.4 IP routing example 2 with more routers
2621A
F0/0
E0
2501A
S0
S0
E0
2501B
S1
S0 E0
2501C