Network Devices are the devices that connects two or more LANS, or segments with each other or with the Internet. These connecting devices can operate in different layers of the Internet model. Some of them are: 

1. Network Interface Card (NIC)

A Network Interface Card (NIC) is a hardware device through which the computer connects to a network. It works 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.

2. Repeater

Repeaters are used to extend LAN. It has only two ports and can connect only two segments of the network. It retimes and regenerates the signals to proper amplitudes and sends them to the other segments. Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data. 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. It can extend the physical length of a LAN.

Repeaters require a small amount of time to regenerate the signal. This can cause a propagation delay which can affect network communication when there are several repeaters in a row. Many network architectures limit the number of repeaters that can be used in a row. Repeaters work only at the physical layer of the OSI network model. 

3. Bridge

Bridge is used to connect two LAN segments like a repeater; it forwards complete and correct frames to the other segment. It works 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) address contained in the frame.  A bridge has a filtering capabilities i.e. it can check the destination address of a frame and decide if the frame should be forwarded or dropped.

A bridge reads the outermost section of data on the data packet, to tell where the message is going. It reduces the traffic on other network segments, since it does not send all packets. Bridges can be programmed to reject packets from particular networks. Bridges forward all broadcast messages. Only a special bridge called a translation bridge will allow two networks of different architectures to be connected. Bridges do not normally allow connection of networks with different architectures. 

4. Hub

It is a device that centrally connects devices in a computer network.  There are two types of a hub i.e. active hub which is a multiport repeater that is used to create connections between stations in a physical star topology; and passive hub which is just a connector that connects the wires coming from different branches. 

Every computer is directly connected with the hub.  When data packets arrives at hub, it broadcast them to all the LAN cards in a network and the destined recipient picks them and all other computers discard the data packets.  Hub has four, eight, sixteen and more ports and one port is known as uplink port, which is used to connect with the next hub. Hubs work at the physical layer of the OSI (Open System Interconnection) model. The device is a form of multiport repeater. Repeater hubs also participate in collision detection, forwarding a jam signal to all ports if it detects a collision.

5. Switch

Like hub, switch also connects multiple computers in a network or different segments of the same network. Switches work at the Data Link Layer of the OSI reference model. Hence, switches consider data as frames and not as signals. Unlike the hubs, a switch does not broadcast the data to all the computers; it sends the data packets only to the destined 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.

6. Router

A router is a communication device that is used to connect two logically and physically different networks, two LANs, two WANs and a LAN with WAN.  The main function of the router is to sorting and the distribution (i.e. routing) of the data packets to their destinations based on their IP addresses. Routers provides the connectivity between the enterprise businesses, ISPs and in the internet infrastructure, router is a main device. Every router has routing software, which is known as IOS.  Router operates at the network layer of the OSI model.  Router does not broadcast the data packets. Routers connect two or more logical subnets, each having a different network address.

A router determines which way is the shortest or fastest in a network, and routes packets accordingly based on the IP addresses.

7. Gateway

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.  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.