Chapter 1: Introducing Layer 3 Routers

Exam Objectives

Describing network OSI Layer 3 and how it relates to Layer 3 routers

Describing the purpose of a Layer 3 router

Differentiating a Layer 3 router from a Layer 2 switch

Describing basic Layer 3 router functions

Recall that Layer 3 in the Open Systems Interconnection (OSI) and TCP/IP network models is the network layer. The main function of network layer devices and protocols is to route data packets between hosts. Routing means choosing the best route to send packets between two hosts. The hosts can be next to each other or across the globe. Whereas the data link layer (Layer 2) routes data frames locally between two devices in the same LAN, the network layer (Layer 3) routes data packets between two devices in separate LANs. In other words, the data link layer sends data within the same LAN, whereas the network layer routes data between LANs.

Layer 3 — Network Layer Review

The network layer assigns logical addresses (IP addresses) to all devices in the network to be able to identify each source host, each destination host, as well as each network through which packets need to be routed. Logical addresses are assigned at the network protocol level as opposed to physical addresses, which are assigned on a physical device, such as a network card.

The network layer does the following:

Receives each data segment from the transport layer on the sending host

Wraps up the segment in a data packet along with global routing data ...

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