Learning How to Go to the Default Post Office
The first time Hannah tries to build the Ethernet header, she
will need more information. To finish building the Ethernet header, Hannah needs
to put R1's Ethernet address in the header as the destination address. The
problem is that Hannah knows the default gateway IP addressin this case
220.127.116.11but she doesn't know R1's Ethernet address on that same interface.
A host can learn its default gateway IP address in two ways. In
some cases, the default gateway IP address is simply typed into the right place
in the PC's configuration. In other cases, the host learns the default gateway
IP address dynamically, using the Dynamic Host Configuration Protocol (DHCP). IP
hosts use DHCP to automatically discover their own IP address, as well as the IP
address of their default router. To keep the coverage focused on the basics,
this chapter doesn't cover the details of how DHCP works. Regardless of how
Hannah knows her default gateway IP address, she must know it.
In contrast, a host does not knowat least ahead of timethe MAC
address used to reach the default router (0200.3333.3333 in this example). To
learn the MAC address information, Hannah uses ARP. ARP is a TCP/IP protocol that can
specifically help solve the problem of knowing an IP address and needing to know
the corresponding LAN MAC address. To see how ARP works, examine Figure 11-6, which shows what Hannah and R1
know before Hannah uses ARP.