Computer Fundamental
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Network Devices

Network Devices

Network Interface Card

• A Network Interface Card (NIC) is a hardware device through which the computer connects to a network.
• NIC is an expansion card, it can be either ISA or PCI, or can be on-board integrated on a chipset. NIC has an appropriate connector to connect the cable to it. NIC for different LAN are different (NIC for token ring is different from NIC for Ethernet).
• NIC work at both the data link layer and physical layer of the OSI reference model.
• At the data link layer, NIC converts the data packets into data frames, adds the Media Access address (MAC address) to data frames. At the physical layer, it converts the data into signals and transmits it across the communication medium. The MAC address is a globally unique hardware number present on the NIC and is specified by the NIC manufacturer.
• NIC depends upon the configuration of the computer, unlike hub or switches that perform independently.


• Repeaters are used to extend LAN. It has only two ports and can connect only two segments of a network. Multiple repeaters can be used to connect more segments. (Segment is a logical section of the same network).
• Repeaters operate at the Physical layer of OSI reference model.
• They are useful when computers in a network are located far away from each other.
• Repeaters amplify the signal so that the signal is as strong as the original signal. They can thus extend the reach of a network.
• Repeaters cannot be used if multiple computers need to be interconnected or multiple segments need to be interconnected.
• Repeaters cannot identify complete frames. Thus, in addition to the valid transmissions from one segment to another, repeater also propagates any electrical interference occurring on a segment to other segment.


• Bridge is used to connect two LAN segments like a repeater; it forwards complete and correct frames to the other segment. It does not forward any electrical interference signals to the other segment.
• Bridge forwards a copy of the frame to the other segment, only if necessary. If a frame is meant for a computer on the same segment, then bridge does not forward a copy of the frame to other segment.
• Bridge connects networks that use different protocol at the Data Link Layer. The frame format of data in the two networks is different. The bridge converts the frame format before transmitting data from one network to another, with translation software included in the bridge.
• A bridge is also used to divide a network into separate broadcast domains to reduce network traffic while maintaining connectivity between the computers.


• It is like a repeater with multiple ports. But, hub does not amplify the incoming signal.
• Hub operates at the Physical layer of OSI reference model, hence treats data as a signal.
• Hubs are used to connect multiple segments of the same network.
• Hubs are also used to connect computers to network that use Star topology.
• The port on the hubs can also be used to connect another hub, switch, bridge or router.
• Hubs increase the network traffic because they broadcast data to all the device connected all the ports of the hub.
• It is preferable to use a hub in a small LAN having about 8–10 computers connected to it.


• Like hub, switch also connects multiple computers in a network or different segments of the same network. A hub simulates a single segment that is shared by all computers attached to it (hub transmits the data to all computers attached to it). In a hub, at most two computers can interact with each other at a given point of time. However, in a switch each computer attached to a switch has a simulated LAN segment.
• Switches work at the Data Link Layer of the OSI reference model. Hence, switches consider data as frames and not as signals.
• A data frame contains the MAC address of the destination computer. A switch receives a signal as a data frame from a source computer on a port, checks the MAC address of the frame, forwards the frame to the port connected to the destination computer having the same MAC addresses, reconverts the frame back into signal and sends to the destination computer. (Switching is a technique that reads the MAC address of the data frame and forwards the data to the appropriate port). Switches, thus, regenerate the signals.
• Since a switch does not broadcast data, but sends the data from the source computer to the destination computer, a half of the computers attached to the switch can send data at the same time.
• Switch is also referred to as a multi-port bridge. In general, bridges are used to extend the distance of the network, and switches are primarily used for their filtering capabilities to create a multiple and smaller virtual LAN (a LAN segment can be connected to each port of the switch) from a single large LAN.


• Router is used to connect heterogeneous networks.
• A router has a processor, memory, and I/O interface for each network to which it connects.
• A router connects networks that use different technologies, different media, and physical addressing schemes or frame formats.
• A router can connect two LANs, a LAN and a WAN, or two WANs.
• A router is used to interconnect the networks in the Internet.
• Router operates at the Network layer of the OSI model (layer 3).
• Physically, a router resembles a bridge, but is different from a bridge. A router determines which way is the shortest or fastest in a network, and routes packets accordingly. Since it works at the Network layer, it moves packets based on the IP addresses etc. In contrast, a bridge connects two LANs almost permanently.


• Gateway is a generic term used to represent devices that connect two dissimilar networks.
• A gateway at the transport layer converts protocols among communications networks. It can accept a packet formatted for one protocol and convert it to a packet formatted for another protocol, before forwarding it. An application gateway can translate messages from one format to the other.
• A gateway can be implemented in hardware, software, or in both hardware and software. Generally, gateway is implemented by software installed within a router.