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TEHow ARP works
Address Resolution Protocol (ARP) is a very fundamental protocol in computer networking. When a PC wants to send a message over the network, it has to encapsulate the data down the layers of the OSI model. At each layer, it has to fill all header information such as TCP/UDP ports in the layer 4 header, source, and destination IP addresses in the Layer 3 header, and source and destination MAC addresses in the Layer 2 header. If you think about it, all this information is available to the end client except for the destination MAC address. Address Resolution Protocol (ARP) has been introduced to resolve a MAC address based on a given IP address in a local network.
PC1 tries to ping PC3, which is in the same local area network and the same subnet 10.1.1.0/24. When the user executes the command ping 10.1.1.3, PC1 starts encapsulating an ICMP Request (ping) into an Ethernet frame before sending it over the network. Let's look at how PC constructs the protocol data unit (PDU):
At layer 4 - PC1 knows that ping works by sending ICMP Echo Request and waits for ICMP Echo Response. So it sets the protocol at layer 4 to be ICMP with Echo Request flag set. Therefore, everything needed at this layer is available.
At layer 3 - PC1 knows the destination IP address, it is explicitly mentioned by the user in the ping 10.1.1.3 command so it puts it into the destination IP field. PC1 knows its own configured IP address 10.1.1.1 and puts it in the source field. Therefore, everything needed at this layer is available.
At later 2 - PC1 knows its own configured MAC address and put in the source field. BUT there is no way for PC1 to know which end client in the LAN has 10.1.1.3 configured and what is its MAC address. Therefore destination MAC address is not available to PC1 and it has to use ARP in order to get it.
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