In this illustration, we see two desktop computers labeled "Host," with a link-layer switch and a router in between them-the switch is on the left and the router on the right. Each element in this figure (Host, Switch, Router, Host) is associated with a protocol stack. The two hosts have five-layer stacks, which include (from top to bottom): Application, Transport, Network, Link, and Physical. The switch has a two-layer stack, which includes Link and Physical. The router has a three-layer stack, which includes Network, Link and Physical. A blue line begins in the top layer of the host's protocol stack, extends downward through it, goes through the Link layer of the switch's stack, through the Network layer of the router's stack, and upwards through the other host's stack, ending in the Application layer.