Network Devices: The network devices (connecting devices) are divided into five different categories based on the layer in which they operate in a network. NIC (Network Interface Card) is used to enable a network device, such as a computer or other network equipment, to connect to a network. A. Passive Hubs A passive hub is just a connector. It connects the wires coming from different branches. In a star-topology Ethernet LAN, a passive hub is just a point where the signals coming from different stations collide; the hub is the collision point. B. Repeater A repeater is a device that operates only in the physical layer. A repeater receives a signal and, before it becomes too weak or corrupted, regenerates the original bit pattern. The repeater then sends the refreshed signal. A repeater can extend the physical length of a LAN. A repeater does not actually connect two LANs; it connects two segments of the same LAN. C. Active Hub An active hub is actually a multipart repeater. It is normally used to create connections between stations in a physical star topology 25
D. Bridge A bridge operates in both the physical and the data link layer. As a physical layer device, it regenerates the signal it receives. As a data link layer device, the bridge can check the physical (MAC) addresses (source and destination) contained in the frame. One may ask, What is the difference in functionality between a bridge and a repeater? A bridge has filtering capability. It can check the destination address of a frame and decide if the frame should be forwarded or dropped. If the frame is to be forwarded, the decision must specify the port. A bridge has a table that maps addresses to ports. E. Two-Layer Switches A two-layer switch is a bridge, a bridge with many ports and a design that allows better (faster) performance. F. Router router is a three-layer device that routes packets based on their logical addresses (host-to-host addressing). A router normally connects LANs and WANs in the Internet and has a routing table that is used for making decisions about the route. G. Three-Layer Switches A three-layer switch is a router, but a faster and more sophisticated. The switching fabric in a three-layer switch allows faster table lookup and forwarding. H. Gateway A gateway is normally a computer that operates in all five layers of the Internet or seven layers of OSI model. A gateway takes an application message, reads it, and interprets it. This means that it can be used as a connecting device between two internetworks that use different models. 26
Internetworking: Figure below show the basic Local Area Network (LAN) that using a hub. This is one collision domain and one broadcast domain network. It is likely that at some point of network design you will have to break up one large network into a bunch of smaller ones because user response will have dwindled to a slow crawl as the network grew and grew. And with all that growth, your LAN s traffic congestion has reached epic proportions. The answer to this is breaking up a really big network into a number of smaller ones something called network segmentation. You do this by using devices like routers, switches, and bridges. Figure below displays a network that s been segmented with a switch so each network segment connected to the switch is now a separate collision domain. But make note of the fact that this network is still one broadcast domain. 27
Hubs do not segment a network; they only connect network segments together. Keep in mind that the hub used in Figure above just extended the one collision domain from the switch port. Hub only can pull out one collision domain as of the switch port. Collision Domain illustrates a network structure in which one specific device sends a packet on the network segment, making each other device on that same segment to listen to it. On that time, a unusual device attempts to answer, directing to a collision, after that both devices must retransmit, one at a time. These types of conditions can be found in hubs environment. On the switch, each port of switch has its own collision domain. Now routers are used to connect networks together and route packets of data from one network to another. Routers, by default, break up a broadcast domain the set of all devices on a network segment that hear all the broadcasts sent on that segment. Figure below shows a router in our little network that creates an internetwork and breaks up broadcast domains. The network in Figure above is a pretty cool network. Each host is connected to its own collision domain, and the router has created two broadcast domains. And don t forget that the router provides connections to WAN services as well. Breaking up a broadcast domain is important because when a host or server sends a network 28
broadcast, every device on the network must read and process that broadcast unless you ve got a router. 29