ses. In telecom, switching is the act of making a connection between two parties. In routing, switching is the process of forwarding packets from one interface to another within a router. Switches differ from hubs in one very fundamental way: a signal that comes into one port is not replicated out every other port on a switch as it is in a hub (unless, as we'll see, the packet is destined for all ports). While modern switches offer a variety of more advanced features, this is the one that makes a switch a switch. Figure 2-7 shows a switch with paths between Ports 4 and 6, and Ports 1 and 7. The beauty is that frames can be transmitted along these two paths simultaneously, which greatly increases the perceived speed of the network. A dedicated path is created from the source port to the destination port for the duration of each frame's transmission. The other ports on the switch are not involved at all. Port 4 Port 5 Port 6 Port 7 Port 0 Port 1 Port 2 Port 3 Figure 2-7. A switch forwards frames only to the ports that need to receive them So, how does the switch determine where to send the frames being transmitted from different stations on the network? Every Ethernet frame contains the source and des- tination MAC address for the frame. The switch opens the frame (only as far as it needs to), determines the source MAC address, and adds that MAC address to a table if it is not already present. This table, called the content-addressable memory table (or CAM table) in CatOS, and the MAC address table in IOS, contains a map of which MAC addresses have been discovered on which ports. The switch then determines the frame's Switches destination MAC address and checks the table for a match. If a match is found, a path is created from the source port to the appropriate destination port. If there is no match, the frame is sent to all ports. When a station using IP needs to send a packet to another IP address on the same network, it must first determine the MAC address for the destination IP address. To accomplish this, IP s