OSI for CCNA Layer 3: Network
The CCNA exam covers data packets; the network layer routes data packets across networks that link the sending and the receiving host. In a nutshell, the network layer does the following:
Chooses the best route to send packets between hosts.
Assigns logical addresses to all devices in the network to be able to identify each source host and each destination host, as well as each network through which packets need to be routed.
Logical addresses are assigned at the network protocol level. Physical addresses are assigned on a physical device, such as a network card.
Receives each data segment from the transport layer on the sending host and wraps it up in a data packet along with routing data. The packet is sent down to the data link layer to send it over the network physical medium.
On the receiving host, the network layer unwraps the packet received to extract the data segment and sends it up to the transport layer.
Several protocols operate at the network layer, such as IP, IPX, AppleTalk, and SNA, but the CCNA test is only concerned with IP.