Layer 4 devices

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Prerequisite – TCP/IP Model, Network Devices 

1. Physical Layer – Physical layer of TCP/IP model is responsible for physical connectivity of two devices. Some of the devices used in Physical layers are, 
 

• Hubs: 
  Hubs are devices commonly used to connect segments of a LAN. It contains multiple input/output ports. when signal is at any input port, this signal will be made at all output ports except the one it is coming from.
• Cables: 
  In Wired network architecture (e.g Ethernet), cables are used to interconnect the devices. some of the types of cables are coaxial cable, optical fiber cable, and twisted pair cable.
• Modem:                                                                                                                                                                                                         Modem stands for MOdulator/DEModulator. A modem converts digital signals generated by the computer into analog signals which, then can be transmitted over cable line and transforms incoming analog signals into digital equivalents.
• Repeaters: 
  Repeaters are used in transmission systems to regenerate analog or digital signals distorted by transmission loss. Analog repeaters can only amplify the signal whereas a digital repeaters can reproduce a signal to near its original quality.

2. Data Link Layer – Data Link layer is responsible to transfer data hop by hop (i.e within same LAN, from one device to another device) based on the MAC address. Some of the devices used in Data Link layer are, 
 

• Bridges: 
  A bridge is a type of computer network device that provides interconnection with other networks that use the same protocol, connecting two different networks together and providing communication between them.
• Switch: A network switch is a multiport network bridge that uses MAC addresses to forward data at the data link layer (layer 2) of the OSI model. Some switches can also forward data at the network layer (layer 3) by additionally incorporating routing functionality. Such switches are commonly known as layer-3 switches or multilayer switches.
• Network Interface Card: 
  Network interface card is an electronic device that is mounted on ROM of the com that connects a computer to a computer network, usually a LAN. It is considered a piece of computer hardware. Most modern computers support an internal network interface controller embedded in the motherboard directly rather than provided as an external component.

3. Network Layer – The network layer is responsible for creating routing table, and based on routing table, forwarding of the input request. Some of the Devices used in Network Layer are, 
 

• Routers: 
  A router is a switch like device that routes/forwards data packets based on their IP addresses. Routers normally connect Local Area Network (LANs) and Wide Area Network (WANs) together and have a dynamically updating routing table based on which they make decisions on routing the incoming packets.
• Brouters: 
  A bridge router or brouter is a network device that works as a bridge and as a router. The brouter routes packets for known protocols and simply forwards all other packets as a bridge would. Brouters operate at both the network layer for routable protocols (or between network with different data link layer protocol ex. one is running on ethernet (802.3) and other network is running on Token ring (802.5)) and at the data link layer for non-routable protocols (or both network are using same data link layer protocol).

Figure: Typical interconnection of Router, Switch, Hub and Bridge 

4. Transport Layer – Transport layer is responsible for end-to-end communication (or process-to-process communication). Some of the transport layer devices are, 
 

• Gateways: 
  In computer networking, a gateway is a component that is part of two networks, which use different protocols. The gateway is a protocol converter which will translate one protocol into the other. A router is a special case of a gateway.
• Firewall: 
  A firewall is a system designed to prevent unauthorized access to or from a private network, some of the functionalities of firewall are, packet filtering and as a proxy server.

5. Application Layer – Application layer is the top most layer of TCP/IP Model that provides the interface between the applications and network. Application layer is used exchange messages. Some of the devices used in Application layer are, 

• PC’s (Personal Computer), Phones, Servers
• Gateways and Firewalls