OSI Network Layer Functions 103
OSI Network Layer Functions
On the CCNA exam, the two key functions for any Layer 3 protocol are routing and addressing.
These two functions are intertwined and are best understood by considering both at the same
time.
Network layer (Layer 3) addressing will be covered in enough depth to describe IP, IPX, and
AppleTalk addresses. Also, now that data link and network layer addresses have been covered
in this chapter, this section undertakes a comparison of the two as well.
Routing
Routing can be thought of as a three-step process, as seen in Figure 3-17. Thinking about
routing in these three separate steps helps make some of the details more obvious. However,
most people will not think of routing as a three-step process when going about their normal
jobs--this is just a tool to make a few points more clearly.
Figure 3-17
Three Steps of Routing
As illustrated in Figure 3-17, the three steps of routing include the following:
Step 1
Sending the data from the source computer to some nearby router
Step 2
Delivering the data from the router near the source to a router near
the destination
Step 3
Delivering the data from the router near the destination to the end
destination computer
Step 1: Sending Data to a Nearby Router
The creator of the data, who is also the sender of the data, decides to send data to a device in
another group. A mechanism must be in place so that the sender knows of some router on a
common data link with the sender to ensure that data can be sent to that router. The sender sends
a data link frame across the medium to the nearby router; this frame includes the packet in the
Bunches
of
Routers
R1
R2
Fred
Barney
Step 2
Step 1
Step 3
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