Internetworking Models ,Network Layer in detail.

The Network Layer :

The Network layer is answerable for routing through an internetwork and for network addressing. This means that the Network layer is answerable for transporting traffic between devices that aren't locally connected. Routers, or Sender Receiver other layer-3 devices, are specified at the Network layer and provide the routing services in an internetwork.

When a packet is received on a router interface, Its check destination IP address. If the packet isn't destined for the router, then the router will look up the destination network address within the routing table. The entry for the destination network isn't found within the routing table, the router drops the packet. 

Two kinds of packets are used at the network layer, data and route updates. 

Data packets are used to transport user data through the inter-network, and protocols used to support data traffic are called routed protocols. Routed protocols examples are IP and IPX. 

Route update packets are used to update neighbor routers about networks connected to routers in the internetwork. Protocols that send route update packets are called routing protocols, For examples are RIP, EIGRP, and OSPF, to name a few. Routing update packets are used to help build and maintain routing tables on each router. The routing table utilized in a router includes the following information,

Network addresses: Protocol-specific network addresses. A router must maintain a routing table for the single routing protocols because of each routing protocol keeps track of a network with a different addressing scheme. 

Interface: The exit interface a packet can take once destined for a particular network.

Metric: The distance to the remote network. Different routing protocols use completely different strategies of computing this distance. 

Routers break up broadcast domains. This means, by default, that broadcasts aren't forwarded through a router. This is good. Routers also break up collision domains, but this can also be accomplished through layer-2 switches. Each interface in a router is a separate network and should be allotted unique network identification numbers. Each host on the network connected to its router must use that very same network range. Some points about routers that we must remember.

  • Routers, won't forward any broadcast or multicast packets.
  • Routers use the logical address during a network layer header to see the next-hop router to forward the packet to.
  •  Routers will use access lists, created by an administrator, to control security on packets trying to either enter or exit an interface. 
  • Routers will offer layer-2 bridging functions if required and may at the same time route through a similar interface. Layer-3 devices (routers in this case) provide connections between Virtual LANs (VLANs). 
  • Routers will offer Quality of Service (QoS) for specific kinds of network traffic.

No comments