IP address of 172.16.10.1) must be known. Why? So the packet can be
handed down to the Data Link layer, framed, and sent to the router's
interface connected to the 172.16.10.0 network. Hosts communicate
only via hardware addresses on the local LAN.
gateway has been already resolved to a hardware address:
for framing. (The hardware destination address is also handed
down with that packet.)
host, an ARP broadcast is sent out onto the local network to
search for the hardware address of 172.16.10.1. The router
responds to the request and provides the hardware address of
Ethernet 0, and the host caches this address. The router also
caches the hardware address of Host A in the ARP cache.
Data Link layer, the LAN driver is used to provide media access via
the type of LAN being used (in this example, Ethernet). A frame is
then generated, encapsulating the packet with control information.
Within that frame are the hardware destination and source addresses,
plus an Ether-Type field that describes the Network layer protocol
that handed the packet to the Data Link layer--in this case, IP. At the
end of the frame is something called a Frame Check Sequence field
(FCS) that houses the answer to the cyclic redundancy check (CRC).
layer to be put on the physical medium (in this example, twisted-pair
wire) one bit at a time.
frame. They each run a CRC and check the answer in the FCS field. If
the answers don't match, the frame is discarded. If the CRC matches
(which, in this example, is the router's interface Ethernet 0), then the
hardware destination address is checked to see if it matches too. If it's
a match, then the Ether-Type field is checked to find the protocol used
at the Network layer.