Hub. The center of a star configuration. The wires connect the hub to each other network device, including workstations, servers, printers, routers, switches, or other hubs. Every device on the hub must operate at the same data transmission rate. Because a hub is not an intellegent device (it sends every received packet to every device on the LAN), the current trend is to replace hubs by switches. A rule of thumb is that any two devices on a LAN should not be connected through more than five hubs.
Bridge. A bridge is used to interconnect different LAN segments. It builds address tables dynamically by comparing the source and destination addresses of the packets being passed to it. A bridge can reduce network traffic by dropping frames that need not be forwarded. Traditionally, a bridge only connected two LAN segments of the same type. However, some modern bridges can be used to connect LANs using different media.
Switch. A switch is a combination of a hub and a bridge. It can connect workstations like a hub, but it can also filter out frames belonging to a different segment of the LAN. Whereas a bridge only has two ports, a switch can have multiple ports. Unlike a hub, a switch forwards a packet out through a single port. A switch can connect LANs that operate at different bit rates.
Router. Of all the packet forwarding discussed in these definitions, routers are the most intellegent. They can connect LANs using different protocols. Routers generally forward their packets using Layer 3 addresses such as IP, Novell's IPX protocol, or AppleTalk. The routing algorithm then takes the Layer 3 address and decides the "best" way to get this packet to its destination address by Level 2 device-to-device hops.
Gateway. A gateway is a router placed between a LAN and a WAN such as the internet.
Firewall. A gateway that performs security functions such as access control, content screening such as virus checking or checking for junk mail. It can also keep a log that records connection requests and server activity to aid in tracking down security breaches. A firewall operates at all of the seven OSI layers.
Repeater. A device that strengthens attenuated signals. In an Ethernet LAN, repeaters are usually required every 500 feet.
Server. A device that stores software, data sets, databases, and other files that multiple users need to access. The server can also perform tape backups of individual workstations.
Channel Service Unit (CSU). The CSU is placed between the customer and the ISP to equalize the digital signal, filter the transmitted and received waveforms, and interact with both the customer's and ISP's testing equipment.