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The network layer is the third layer of the OSI model and responsible for routing data packets. It includes routers, protocols, and algorithms for packet switching and QoS. IP addresses are used for routing information on the Internet.
The network layer refers to layer 3 of the seven-layer Open Systems Interconnection (OSI) model for network architecture. Algorithms and protocols in the network layer are responsible for identifying the optimal path to reliably route and deliver data packets from source networks to destination networks. As a component of the OSI conceptual framework, the network layer answers service requests from layer 4, the transport layer, and sends service requests to layer 2, the data link layer. Basically, it consists of equipment, primary routers, and mechanisms such as the Internet Protocol (IP), required to transmit digitally encoded signals from source hosts across digital networks, such as the Internet, to destination hosts. Key functions of the network layer include addressing, congestion control, error handling, internetworking and packet sequencing, routing, and forwarding.
Addressing and routing to ensure a required level of Quality of Service (QoS) are the main functions of the Network Layer. It contains routing and IP algorithms, protocols, and the Address Resolution Protocol (ARP). Data is transported across digital networks via routers and switches that use protocols to encapsulate it in a packet and algorithms that identify the best transmission path. This is known as packet switching.
Layer 3, or network layer, switching occurs when a packet arrives at one router interface and is forwarded to another. Protocols that can be present in the network layer include Datagram Delivery Protocol (DDP), Internet Control Message Protocol (ICMP), Internet Group Message Protocol (IGMP), and Internet Protocol Security (IPsec). Commonly used protocols that route information across networks include Routing Information Protocol (RIP), Open Shortest Path First (OSPF), Interior Gateway Routing Protocol (IGRP), Border Gateway Protocol (BGP), and Distance Vector Multicast Routing Protocol (DVMRP) .
For routing information on the Internet, IP addresses are included in the headers of network layer messages, thus providing routers with the information they need to decide how to handle the flow of data packets. The destination IP address of the packet is checked when it is received on a router interface. If the router is not the destination of the packet, the router will look up the network address in its routing table. An outgoing interface is selected and the packet is sent to the interface to be framed and forwarded on the local network. The router drops the packet if the destination network entry is not found in the routing table.
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