Unicast

Unicast
Unicast
is used for direct host-to-host communication. When the layer 3 protocol data unit
(PDU, or packet) is formed, two layer 3 IP addresses are added to the IP header. These are the
source and destination IP addresses. They specify a particular originating and receiving host.
After the layer 3 PDU is formed, it is passed to layer 2 to create the layer 2 PDU, or frame. The
frame consists of all the previous layers’ headers in addition to the layer 2 header. With an
Ethernet frame, for example, the two 48-bit source and destination MAC addresses are specified
in the layer 2 header. Other protocols such as IEEE 802.5 (Token Ring) and FDDI also have
headers that contain specific host source and destination addresses.
Unicast communication is used when two hosts need to exchange data with only each other
and are not concerned with sharing the data with everyone. A MAC address must
uniquely
identify a host. No two MAC addresses on a single network can be the same. Therefore, unicast
capitalizes on the unique MAC address of each host. With the specific address, any source host
should be able to contact the destination host without confusion.
One of the caveats with unicast communication is that the source host must know or be
able to learn what every destination MAC address is for every station it wishes to communicate
with. In order to figure out which MAC address the source should send frames to,
it uses an ARP request, as explained in the following section. The normal operation is that
the host has a default gateway assigned for use when the logical destination address does
not reside on the same subnet as the source host. Figure 19.1 depicts how unicast traffic
works on the same subnet.
Of course, unicast traffic may differ inside an internetwork interconnected by
routers. In those circumstances, you will remember that the transmitting client
needs to know the IP (and MAC) addresses of the default gateway.