Network Layer

Moving down the OSI model to layer 3 we have the network layer. This layer's main responsibility is addressing and routing packets to their proper destination. To accomplish this, the network layer adds information to the header of packets that details what computer a packet is intended for. The packet is then passed on to the lower layers where it will eventually be placed on the wire. As a packet traverses a network, each system examines the header of the packet to see where the packet is addressed for and how to route it. The protocols at this layer will also help in routing the packet to it's destination. Routing tables are built and maintained which detail the layout of the network. Some routing protocols are able to determine the best path for a packet to take based on multiple factors including distance and transmission rate. So when a packet needs to be sent the routing protocol will check it's routing table, add the necessary information, and send it on its merry way.

Protocols that work on this layer do nothing to ensure delivery. They rely on the protocols of the transport layer to provide that functionality (if the protocol used by that layer even provides that option). Protocols at the network layer are solely concerned with routing and addressing. The most well known protocol at this layer is the Internet Protocol (IP). As the name suggests, this is the core protocol that much of the internet functions on. Other protocols include Internet Control Message Protocol (ICMP), Border Gateway Protocol (BGP), Routing Information Protocol (RIP), Open Shortest Path First (OSPF), and Internet Group Management Protocol (IGMP).