Thursday, November 21, 2013

Network Layer

Right now we discuss about Network Layer.
First of all, what is Network Layers? what is the main function of this layer?
The Network layer(usually called layer 3) manages devices addressing (logical addressing), tracks the location of devices on the network, and determines the best way to move data (routing data).
The devices that working in this layer of course is router. :)
Below is the flow for networking:
First, when a packet is received on router interface, the Destination IP address is checked. If the packet isn’t destined for that specific router, it will look up the destination network address in routing table.
Once the router chooses an exit interface, the packet will be sent to that interface to be framed and sent out on the local network. If the router can’t find an entry for the packet’s destination network in the routing table, the router drops the packet.
Two types of packets are used at the Network Layer: data and route updates.
Data packets
Used to transport user data through the internetwork. Protocol used to support data traffic are called routed protocols.
example: IP and IPX.
Route update packets
Used to update neighboring routers about the networks connected to all routers within the internetwork. Protocol that send route update packets are called routing protocols.
example: RIP,EIGRP,and OSPF.
The Routing table used in a router includes the following information:
Network address
A route must maintain a routing table for individual routing protocols because each routing protocol keeps track of a network with different addressing scheme.
The exit interface a packet will take when destined for a specific network
The distance to the remote network.Different routing protocols use different ways of computing this distance.
hop count
The number of routers a packet passes through en route to a remote network.
router break up broadcast domains and collision domains. (broadcast aren’t forwarded through a router).
To make u more understand more about broadcast domain and collision domain
below is the video of explanation broadcast and collision domains.

To make u more easier, i will review the point that you should remember in this section:
1. Routers, by default, will not forward any broadcast, multicast packets
2. Routers use the logical address in a Network layer header to determine the next hop router to forward the packet to
3. Routers can use access lists, created by an administrator, to control security on the types of packets that are allowed to enter or exit an interface
4. Routers can provide layer-2 bridging functions if needed and can simultaneously route through the same interface.
5. layer 3 devices provide connections between virtual LANs (VLANs).
6. Routers can provide quality of service (QoS) for specific types of network traffic.


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