OSI Reference Model
Layer 3: Network Layer
Now let’s take a look a layer 3--the
domain of routing.
Network Layer: Path Determination
Which path should traffic take through the
cloud of networks? Path determination occurs at Layer 3. The
path determination function enables a router to evaluate the
available paths to a destination and to establish the preferred
handling of a packet.
Data can take different paths to get from a source to a destination.
At layer 3, routers really help determine which path. The
network administrator configures the router enabling it to
make an intelligent decision as to where the router should
send information through the cloud.
The network layer sends packets from source network to destination
After the router determines which path to use, it can proceed
with switching the packet: taking the packet it accepted on
one interface and forwarding it to another interface or port
that reflects the best path to the packet’s destination.
To be truly practical, an internetwork must
consistently represent the paths of its media connections.
As the graphic shows, each line between the routers has a
number that the routers use as a network address. These addresses
contain information about the path of media connections used
by the routing process to pass packets from a source toward
The network layer combines this information about the path
of media connections–sets of links–into an internetwork
by adding path determination, path switching, and route processing
functions to a communications system. Using these addresses,
the network layer also provides a relay capability that interconnects
The consistency of Layer 3 addresses across the entire internetwork
also improves the use of bandwidth by preventing unnecessary
broadcasts which tax the system.
Addressing—Network and Node
Each device in a local area network is given
a logical address. The first part is the network number –
in this example that is a single digit – 1. The second
part is a node number, in this example we have nodes 1, 2,
and 3. The router uses the network number to forward information
from one network to another.
Protocol Addressing Variations
The two-part network addressing scheme extends
across all the protocols covered in this course. How do you
interpret the meaning of the address parts? What authority
allocates the addresses? The answers vary from protocol to
For example, in the TCP/IP address, dotted decimal numbers
show a network part and a host part. Network 10 uses the first
of the four numbers as the network part and the last three
numbers–8.2.48 as a host address. The mask is a companion
number to the IP address. It communicates to the router the
part of the number to interpret as the network number and
identifies the remainder available for host addresses inside
The Novell Internet Package Exchange or IPX example uses a
different variation of this two-part address. The network
address 1aceb0b is a hexadecimal (base 16) number that cannot
exceed a fixed maximum number of digits. The host address
0000.0c00.6e25 (also a hexadecimal number) is a fixed 48 bits
long. This host address derives automatically from information
in hardware of the specific LAN device.
These are the two most common Layer 3 address types.