OSI Network Layer

The network layer of the OSI reference model handles the moving of data between networks of networks, called internetworks.

While the data link layer uses hardware addresses within a single segment of a network, the network layer uses logical addresses and host IDs. The main hardware device operating at the network layer is the router.

The router is responsible for adding a destination network address to every packet it receives. That header contains information on how to reach the different logical network segments the packets need to go to. Through the internetwork, the routers forward the packet accordingly until the packet reaches it's destination, filtering junk along the way, using firewalls and access control lists (ACLs).