Understanding End Devices In The Network Process
The network devices that people are most familiar with are end devices. To distinguish one end device from another, each end device on a network has an address. When an end device initiates communication, it uses the address of the destination end device to specify where to deliver the message.
An end device is either the source or destination of a message transmitted over the network.
Click Play in the figure to see an animation of data flowing through a network.
The figure shows a physical network topology with a block of LAN, a block of InternetWork, and a block of another LAN. From left to right, a LAN has two users an IP phone, a PC, and a server connected to a switch. A physical link connects the LAN switch to an edge router that borders the LAN block and the Internetwork block. The Internetwork block consists of four routers connected in a full mesh topology.
An edge router borders the Internetwork block and a second LAN block. The second LAN block consists of two users, an IP phone, a PC, and a server. When the animation is started a message originates from one of the users in the first LAN and travels from the user, to the switch and to the edge router that borders the Internetwork. At the Internetwork the message is routed through to the other edger router that borders with the second LAN. The message is forwarded into the second LAN, through the switch, and to the destination end user. The text under the graphic reads Data originates with an end device, flows through the network, and arrives at the end device.
Routers are devices that operate at the OSI network layer (Layer 3). As shown in the figure, routers are used to interconnect remote sites. They use the process of routing to forward data packets between networks. The routing process uses network routing tables, protocols, and algorithms to determine the most efficient path for forwarding an IP packet. Routers gather routing information and update other routers about changes in the network. Routers increase the scalability of networks by segmenting broadcast domains.
Image shows four boxes, one at the top of the graphic labeled Home Office and containing a wireless router, a printer connected by a line representing a wired connection, a wireless tablet, and a wireless laptop. A line connects the wireless router to a cable modem, which connects to the second box labeled WAN, containing a cloud labeled Internet and another cloud labeled Cloud. There are two boxes at the bottom of the graphic, one labeled Central and one labeled Branch.
Both boxes contain router icons connected to the Cloud and to the Internet with WAN media shown as red lightning bolts. In the Central box, there are two multilayer switch icons, connected to two LAN switches. There is a server connected directly to the router and four computers connected to the switches. In the box labeled Branch, there are six end devices connected to a switch icon. The six devices are a server, a printer, two IP phones and two computers. Also connected to the LAN switch is a wireless access point. A wireless tablet and a wireless laptop are shown connecting to the wireless access point.
The Router Connection
Routers have two primary functions: path determination and packet forwarding. To perform path determination, each router builds and maintains a routing table which is a database of known networks and how to reach them. The routing table can be built manually and contain static routes or can be built using a dynamic routing protocol.
Packet forwarding is accomplished by using a switching function. Switching is the process used by a router to accept a packet on one interface and forward it out of another interface. A primary responsibility of the switching function is to encapsulate packets in the appropriate data link frame type for the outgoing data link.
Play the animation of routers R1 and R2 receiving a packet from one network and forwarding the packet toward the destination network.
The animation depicts two LAN networks with hosts connected by two routers R1 and R2. A packet is animated traversing the connection from one LAN to the other LAN. The router screens appear FOR R! and R2 showing the matching IP v.4 addresses in the router passing from one network to another.
After the router has determined the exit interface using the path determination function, the router must encapsulate the packet into the data link frame of the outgoing interface.
What does a router do with a packet received from one network and destined for another network? The router performs the following three major steps:
1. It de-encapsulates the Layer 2 frame header and trailer to expose the Layer 3 packet.
2. It examines the destination IP address of the IP packet to find the best path in the routing table.
3. If the router finds a path to the destination, it encapsulates the Layer 3 packet into a new Layer 2 frame and forwards that frame out the exit interface.
As shown in the figure, devices have Layer 3 IPv4 addresses, while Ethernet interfaces have Layer 2 data-link addresses. The MAC addresses are shortened to simplify the illustration. For example, PC1 is configured with IPv4 address 192.168.1.10 and an example MAC address of 0A-10. As a packet travels from the source device to the final destination device, the Layer 3 IP addresses do not change. This is because the Layer 3 PDU does not change. However, the Layer 2 data link addresses change at every router on the path to the destination, as the packet is de-encapsulated and re-encapsulated in a new Layer 2 frame.
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