In the case of a host, this is the path
between the data link layer and the upper layers of the NOS. In the case of a router, it is the
actual path across the network. Figure 1-59 shows Layer 3 devices.
Layer 3 Addressing
Each NOS has its own Layer 3 address format. For example, the OSI reference model uses
a network service access point (NSAP), while TCP/IP uses an IP address. This course
focuses on TCP/IP. Figure 1-60 shows an example of Layer 3 addressing.
# Bytes 8 6 6 2 Variable 4
Ethernet II
Uses "Type"
Here and
Does Not Use
802.2
Sublayers
Vendor
Assigned
IEEE
Assigned
MAC Address
MAC Sublayer - 802.3
Preamble Dest Add Source Add Length Data FCS
0000.0C xx.xxxx
Exploring the Packet Delivery Process 87
Figure 1-59 Layer 3 Devices
Figure 1-60 Layer 3 Addressing
Mapping Layer 2 Addressing to Layer 3 Addressing
For IP communication on Ethernet-connected networks to take place, the logical (IP)
address needs to be bound to the physical (MAC) address of its destination. This process is
carried out by the Address Resolution Protocol (ARP). Figure 1-61 shows an example of
mapping a Layer 2 address to a Layer 3 address.
To send data to a destination, a host on an Ethernet network must know the physical (MAC)
address of the destination.
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