Figure 1.24 shows the physical path that data takes down a sending end system’s
protocol stack, up and down the protocol stacks of an intervening link-layer switch
and router, and then up the protocol stack at the receiving end system. As we discuss
later in this book, routers and link-layer switches are both packet switches. Similar
to end systems, routers and link-layer switches organize their networking hardware
and software into layers. But routers and link-layer switches do not implement all of
the layers in the protocol stack; they typically implement only the bottom layers. As
shown in Figure 1.24, link-layer switches implement layers 1 and 2; routers implement
layers 1 through 3. This means, for example, that Internet routers are capable
of implementing the IP protocol (a layer 3 protocol), while link-layer switches are
not. We’ll see later that while link-layer switches do not recognize IP addresses, they