But what are the roles of the network layer address and the data link layer address when a device is communicating with a device on a remote network? In this example we have a client computer, PC1, communicating with a server, named Web Server, on a different IP network.
Network Addresses
IP addresses indicate the network and device addresses of the source and destination. When the sender of the packet is on a different network from the receiver, the source and destination IP addresses will represent hosts on different networks. This will be indicated by the network portion of the IP address of the destination host.
- Source IP address - The IP address of the sending device, the client computer PC1: 192.168.1.110.
- Destination IP address - The IP address of the receiving device, the server, Web Server: 172.16.1.99.
Data Link Addresses
When the sender and receiver of the IP packet are on different networks, the Ethernet data link frame cannot be sent directly to the destination host because the host is not directly reachable in the network of the sender. The Ethernet frame must be sent to another device known as the router or default gateway. In our example, the default gateway is R1. R1 has an interface and an IP address that is on the same network as PC1. This allows PC1 to reach the router directly.
- Source MAC address - The Ethernet MAC address of the sending device, PC1. The MAC address of the Ethernet interface of PC1 is AA-AA-AA-AA-AA-AA.
- Destination MAC address - When the receiving device is on a different network from the sending device, this is the Ethernet MAC address of the default gateway or router. In this example, the destination MAC address is the MAC address of the R1 Ethernet interface that is attached to the PC1 network, which is 11-11-11-11-11-11.
The Ethernet frame with the encapsulated IP packet can now be transmitted to R1. R1 forwards the packet to the destination, Web Server. This may mean that R1 forwards the packet to another router or directly to Web Server if the destination is on a network connected to R1.
How does the sending device determine the MAC address of the router?
Each device knows the IP address of the router through the default gateway address configured in its TCP/IP settings. The default gateway address is the address of the router interface connected to the same local network as the source device. All devices on the local network use the default gateway address to send messages to the router. After the host knows the default gateway IP address, it can use ARP to determine the MAC address of that default gateway. The MAC address of the default gateway is then placed in the frame.