Layer 3 is the Network layer. This layer is responsible for moving data around a network of networks, known as an internetwork. While the Data Link layer is capable of forwarding data by using hardware addresses within a single segment, the Network layer moves information around an internetwork by using logical network and host IDs. The networks are often heterogeneous; that is, they use a variety of Physical layer media and Data Link protocols. The main appliance working at layer 3 is the router.
Description
The steps are as follows.
1. At the network later, each interface is identified by an address with a network part (1. for this router interface) and a host part (254).
2. When host 1.2 wants to send to host 2.3, the packet must be delivered to the remote network via routers.
3. Networks 1 and 2 are connected via an intermediate network k (9).
4. Router B recognizes that network 2 is directly connected and uses data link protocols to send the packet to host 2.3.
Communications at layer 3 of the OSI model. (Images © 123RF.com.)
At layer 3, each packet is given a destination network address. Routers are configured with information about how to reach these different logical networks. The packet is forwarded, router by router (or hop by hop), through the internetwork to the target network. Once it has reached the destination network, the hardware address can be used to deliver the packet to the target node.
The general convention is to describe PDUs packaged at the Network layer as packets or datagrams and messages packaged at the Data Link layer as frames. Packet is often used to describe PDUs at any layer, however.
It is usually important for traffic passing between networks to be filtered. A basic firewall operates at layer 3 to enforce an access control list (ACL). A network ACL is a list of the addresses and types of traffic that are permitted or blocked.
