Network Layer (Layer 3) tutorials

Data received from Transport Layer 4 "Segment" is formatted into "Packets" to be transmitted to the Data Link Layer 2.

  • It adds appropriate network addressing and delivery of packets called Datagrams.

  • It provides logical addressing which router use for path determination.

  • The Network layer is responsible for routing through an internetwork and for network addressing.

  • Routers, or other layer-3 devices, are specified at the Network layer and provide the routing services in an internetwork.

  • A router at the Network layer follows following steps to ensure proper data transport:

  • Router checks the destination IP address of the incoming packet on the router interface.

  • Packets destined for that router are processed, whereas packets destined for another router must be looked up in the routing table.

  • The router determines an exit interface based on the routing table and sends the packet to the interface for framing and forwarding. If there is no route in the routing table, the packet is dropped by the router.

  • A routing table on a router contains following information:

  • Network Address

  • Interface: Exit interface used to forward packets

  • Metric : Distance to reach a remote network

  • Two types of packet are used at Network Layer 3:

1) Data Packets: Data packets are used to transport data across the internetwork and are supported by IP and IPX protocols.

2) Route Update Packets: Are used to update neighbor router about networks connected to routers in internetwork. It is used to help build and maintain routing tables on each router. Protocols such as RIP, EIGRP, and OSPF are used here.