Network layer, that defines how Internet works or interconnected network functions. It adds headers containing logical address of the device. Network layer is concerned with sending data from one computer to another computer even if it is on a remote network.
In WAN (Wide Are Network ), two end devices are separated by number of networks. Between the two end devices the data has to be passed through a series of intermediate nodes. These nodes are called as Routers.
Routers are special devices, which are capable of making complex routing decisions. Routers evaluate all available paths to a destination and determine which to use. After selecting the best path, router can continue with switching packets.
Network layer provides functional and procedural means of transferring data from source to destination over one or more networks. Routing and forwarding data are the important functions of network layer. Apart from this, it is concerned with physical and logical addressing of device, error detection, flow control, segmentation and sequencing of data. Internet Protocol ( IP ) handles physical and logical addressing of workstations.
Two important aspects required to send data from one device to another are
- Logical address related to source and destination.
- Path through which the data has to be sent to reach the destination.
- Logical Addressing (Host Addressing) : Every host in the network needs to have a unique address which determines where it is. This address will normally be assigned from a hierarchical system, so you can be "Fred Murphy" to people in your house, "Fred Murphy, Main Street 1" to Dubliners, or "Fred Murphy, Main Street 1, Dublin" to people in Ireland, or "Fred Murphy, Main Street 1, Dublin, Ireland" to people anywhere in the world. On the Internet, addresses are known as Internet Protocol (IP) addresses.
- Routing (Message Forwarding) : Moving the data packets through a series in a network is the main function of network layer. This work is done but the device and software which function at network layer like router. The function include handling incoming packets from several sources, and determining their final destination. Network layer encapsulates the message received from the higher layers by placing network layer headers into the datagram (Datagram Encapsulation). Since many networks are partitioned into subnetwork and connect to other networks for wide-area communications, networks use specialized hosts, called gateways or routers to forward packets between networks. This is also of interest to mobile applications, where a user may move from one location to another, and it must be arranged that his messages follow him. Version 4 of the Internet Protocol (IPv4) was not designed with this feature in mind, although mobility extensions exist. IPv6 has a better designed solution.
- Fragmentation : The network layer has to send message to the data link layer for transmission . But the data link layer has limits on the length that can be sent from the network layer. If the packets that has to be sent from the network layer is very long, the network layer has to break into pieces and send each piece to the data link layer.
The information added to the routing table includes
- Network Address : It is protocol specific . It maintains a unique table for each protocol.
- Interface : It is an interface used by router to send information to reach particular network.
- Metric : Metric indicates the cost of the route. And it is determined by the number of routers that must be crossed to reach the destination (hops). This metric is used to decide between multiple matching routers.
Protocols used in this layers
- IPv4/IPv6, Internet Protocol
- DVMRP, Distance Vector Multi-cast Routing Protocol
- ICMP, Internet Control Message Protocol
- IGMP, Internet Group Multi-cast Protocol
- PIM-SM, Protocol Independent Multi-cast Sparse Mode
- PIM-DM, Protocol Independent Multi-cast Dense Mode
- IPsec, Internet Protocol Security
- IPX, Internetwork Packet Exchange
- RIP, Routing Information Protocol
- DDP, Datagram Delivery Protocol
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