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The Network layer provides routing and related functions that enable multiple data links to be combined into an internetwork.  This is accomplished by the logical addressing (as opposed to the physical addressing) of devices.  The network layer supports both connection-oriented and connectionless service from higher-layer protocols. Network-layer protocols typically are routing protocols, but other types of protocols are implemented at the network layer as well.  Routers work at this level and provide the routing services for an internetwork.

Routing a Packet

1. The router receives the packet and looks up the destination IP address.
2. If the packet isn't destined for the router, the router looks for the destination address in the routing table.
3. Once the destination interface is found, the packet will be sent to the interface.
4. At the destination interface, the packet is framed and sent out on the local network.

-There are two types of packets at the Network layer.

Data Packets

-Used to transport user data through internetwork.

-Uses routed protocols such as: IP and IPX.

Router Update Packets

-Used to update neighbor routers about networks connected to routers on the internetwork.

-Routing protocols: RIP, EIGRP, OSPF.

-Builds and maintains routing tables on each router.

Routing Table

Network Address

-Protocol specific network addresses.  A table is maintained for individual routing protocols since each protocol keeps track of a network with a different addressing scheme.

Interface

-The interface the packet is sent out on when destined for a particular network.

Metric

-The distance to the remote network.

-Routers breakup broadcast domains by not forwarding broadcast or multicast packets through a router.  They also breakup collision domains as each interface is a separate network.

-Routers use logical addresses in a network layer header to determine the next hop router to forward the packet to.

-Routers can use access lists to control security on packets entering or leaving an interface.

-Routers can provide layer 2 bridging and can simultaneously route through the same interface

-Routers provide connections between Virtual LANs. (VLANs)

-Routers can provide Quality of Service for specific types of network traffic.