If every host, camera, television, and toaster already has unique physical addresses, why should we assign interface addresses as well?

The physical address identifies the host, while the interface address describes the topological location of the host. The physical address is a permanent, fixed address every other host attached to the same physical network can use to communicate with it. The interface address, on the other hand, tells other devices where the host is connected to the network or where to send packets if they are not attached to the same physical link.

Another way this might be expressed is the physical address is the address on this wire, while the interface address is the host’s location on this network. The meanings of on this wire and on this network have, as with most terms in the computer network, broadened over time. The prep course of this exam is at https://www.acedexam.com/300-425-enwlsd-designing-cisco-enterprise-wireless-networks/

Yet another way to express the difference between the physical and interface addresses is using the idea of network stack layers, a topic that will be considered in more detail in Chapter 6, “Network Models.” The physical address is commonly called a Layer 2 address, and the interface address is often called a Layer 3 address.

Why not make the physical interface match the interface address? There are network systems where both the physical and interface address are the same. The Open Systems Interconnection network protocol suite, which includes Connectionless Network Protocol (CLNP) and the Intermediate System to Intermediate System (IS-IS) protocols, is designed so a single manually assigned address is used for all the interface and physical addresses.

On the other hand, most network protocols, such as IP, assume a host will need to discover interface addresses once it is attached to the network. If the interface address must be configured to create the physical address, the interface address must be configured before the host can communicate at all—not even with an automatic configuration system. Figure 2-6 illustrates the problem.

Figure 2-6 Address Assignment Bootstrap Problem

There are many ways to solve this bootstrap problem, but the simplest is to make certain the physical address of each device attached to a network is globally unique.

Why not make the interface address match the physical address? Because the interface address is topological, it must also be hierarchical. Rather than being a single flat address space, there must be something like the equivalent of a street number, street name, city, etc., so the address can be aggregated or summarized. Without some form of aggregation, the address of every host in the world would need to be known to every other host in the world—a completely unworkable situation.

Instead, just like in physical shipping, a packet is carried toward its destination in stages, with different parts of the interface address used at different places.