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 network.
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 a destination.
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 independent networks.
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 protocol.
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 that network.
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.