6.1. Address Resolution Protocol

Both logical and physical addresses are used for communication on a network. The use of logical addresses allows for communication between multiple networks and indirectly connected devices. The use of physical addresses facilitates communication on a single network segment for devices that are directly connected to each other with a switch. In most cases, these two types of addressing must work together in order for communication to occur.

Consider a scenario where you wish to communicate with a device on your network. This device may be a server of some sort or just another workstation you need to share files with. The application you are using to initiate the communication is already aware of the IP address of the remote host (via DNS, covered in Chapter 7), meaning the system should have all it needs to build the layer 3 through 7 information of the packet it wants to transmit. The only piece of information it needs at this point is the layer 2 data link data containing the MAC address of the target host.