A source device will send a packet based on an IP address. One of the most common ways a source device determines the IP address of a destination device is through Domain Name Service (DNS), in which an IP address is associated to a domain name. For example, www.cisco.com is equal to 209.165.200.225. This IP address will get the packet to the network location of the destination device. It is this IP address that routers will use to determine the best path to reach a destination. So, in short, IP addressing determines the end-to-end behavior of an IP packet.
However, along each link in a path, an IP packet is encapsulated in a frame specific to the particular data link technology associated with that link, such as Ethernet. End devices on an Ethernet network do not accept and process frames based on IP addresses, rather, a frame is accepted and processed based on MAC addresses.
On Ethernet networks, MAC addresses are used to identify, at a lower level, the source and destination hosts. When a host on an Ethernet network communicates, it sends frames containing its own MAC address as the source and the MAC address of the intended recipient as the destination. All hosts that receive the frame will read the destination MAC address. If the destination MAC address matches the MAC address configured on the host NIC, only then will the host process the message.
Figure 1 shows how a data packet, containing IP address information, is encapsulated with data link layer framing containing the MAC address information.
Figure 2 shows how frames are encapsulated based on the technology of the actual link.
How are the IP addresses of the IP packets in a data flow associated with the MAC addresses on each link along the path to the destination? This is done through a process called Address Resolution Protocol (ARP).