By Kailin AchesonThe previous two parts of this, component I and component II, spanned the seven layers that the open up Systems Interconnection (OSI) model. This installment will certainly cover the four layers the the Transmission control Protocol/Internet Protocol (TCP/IP) model and also contrast the TCP/IP model with the OSI model.

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The TCP/IP model and also the OSI version are connected in that they are layered referral models that represent how data and information space passed between various devices. But they are various in that the TCP/IP model has 4 layers (Application, Transport, Internet, and Network Access) and also the OSI model has actually seven great (Physical, Data Link, Network, Transport, Session, Presentation, and Application). The adhering to graphic shows how the class relate come one another when compared side through side:


As you can see, the TCP/IP model has actually fewer layers, which in some ways overlap the great of the OSI model. Conversely, the OSI design emphasizes particular layers and was structured v the layers together the primary focus, the TCP/IP version was structured with architectural values as the major focus.

To refresh her memory on the class of the OSI model, inspect out component I (which consists the Physical, Data Link, Network, and Transport layers) and part II (which covers the Session, Presentation, and also Application layers). Ns won"t get in an exhaustive discussion of each layer; what friend see below will be a brief an overview of every layer for this reason you have an idea of the basics.

The application Layer

The TCP/IP applications layer, also called the process layer, is the optimal layer, closest to you, the user, and also is responsible for transferring data and also network solutions to the details applications top top a computer for i beg your pardon they room intended. The application layer is likewise responsible for the compression and also encryption the data and the creation, maintenance, and termination of sessions as soon as required. Some usual protocols that run at the TCP/IP applications layer include document Transfer Protocol (FTP), basic Mail transfer Protocol (SMTP), Domain Name system (DNS), Telnet, and Hypertext transfer Protocol (HTTP). The application layer the the TCP/IP model coincides to the applications layer (Layer 7), the Presentation layer (Layer 6), and the session layer (Layer 5) of the OSI model.

The deliver Layer

The TCP/IP deliver layer, additionally called the Host-to-Host layer, is responsible for the shipment of data in between the net layer and also the application layer, error control and recovery, circulation control, jam control, and data segmentation. The common protocols that run at the TCP/IP transport layer room Transmission manage Protocol (TCP) and User Datagram Protocol (UDP). The tools that commonly operate at the transfer layer space network devices, or gateways. The move layer of the TCP/IP model coincides to the move layer (Layer 4) that the OSI model.

The web Layer

The TCP/IP net layer, also known together the Internetwork layer, is responsible for specifying logical addressing and delivering packets. The key protocol provided at this layer, internet Protocol, supplies the resource and location addresses in order come facilitate the movement of data between the Network accessibility layer and also the deliver layer; the data flows from one node ~ above the network to the next node in a path relocating toward the final destination. The two other common protocols that operate at the TCP/IP internet layer room derivatives that IP: IP variation 4 (IPv4) and also IPv6. The gadgets that frequently operate at the net layer room routers. The internet layer the the TCP/IP model coincides to the Network layer (Layer 3) the the OSI model.

The Network accessibility Layer

The TCP/IP Network access layer, likewise referred to together the link layer or Network interface layer, specifies how data will be yielded over the physics network and also which protocols are appropriate for the delivery. This layer is also responsible because that arbitration and also error discovery. The Network accessibility layer deserve to at times be separated right into two various layers: the Data link layer and also the physical layer. Some typical protocols that operate at the TCP/IP Network accessibility layer include attend to Resolution Protocol (ARP) and also Point-to-Point Protocol (PPP). The gadgets that operate at the Network access layer include Layer 2 switches and also bridges (operating top top the Data attach layer) and also Layer 1 hubs and also repeaters (operating ~ above the physical layer). The Network access layer the the TCP/IP model synchronizes to the Data attach layer (Layer 2) and also the physics layer (Layer 1) that the OSI model.

Inter-Layer and also Intra-Layer Communication

Now the you have read the basics of every of the 4 TCP/IP layers, let"s take it a brief look at how they interact and how data moves between them. Lot like the OSI model, a class of the TCP/IP version can connect with the great directly over it or the layer directly below it (inter-area communication) or with the layer directly across from the on a connected device (intra-area communication). The gadgets that run at each layer are listed below:

Application layer: HostsTransport layer: GatewaysInternet layer: RoutersNetwork access layer: class 2 switches and bridges (Data link layer) and also Layer 1 hubs and repeaters (Physical layer)

A hub operates in ~ the Physical layer division of the Network accessibility layer. A hub, i beg your pardon cannot make forwarding decisions concerning the data the receives, merely receives, amplifies, and also regenerates signals in between ports. A hub can only obtain a signal comprise a sequence of bits and also then transmit the very same bits on all its other ports to a class 2 move or bridge. Due to the fact that the tools at this layer have the right to only receive and also retransmit data, no Protocol Data Unit (PDU), such as a packet or a segment, exists at this layer.

A great 2 switch or leg operates at the Data attach layer department of the Network access layer. The present of bits a great 2 gadgets receives native the physics layer becomes well-known as a structure PDU at the Data connect layer. A great 2 switch or bridge deserve to receive a present of bits from the physics layer, verify that the whole frame has arrived safely, and check the integrity of the data through performing a calculation on the obtained data and also comparing the result to the Frame examine Sequence (FCS). If the frame is verified, the layer 2 device can front the structure to the appropriate destination based upon the Media accessibility Control (MAC) attend to and the info in the contents Addressable storage (CAM) table.

A router operates in ~ the Internet layer. The structure a router receives from the Network access layer becomes well-known as a packet PDU at the net layer. The router deserve to verify that the IP header information within the packet has remained intact and can identify whether the packet has been fragmented during the distribution process. The router can then make a forwarding decision based on the destination IP attend to in a packet"s header and the entries in the router"s very own routing table.

Gateways operate at or above the Transport layer; an example of a gateway is a voice gateway that have the right to accept an ISDN-PRI trunk user interface that provides Q.931 indigenous the phone agency and translate it to the H.323 signaling protocol provided by Voice over IP (VoIP) call systems. The packet a gateway receives from the net layer becomes well-known as a flow PDU in ~ the transfer layer. A gateway obtain a packet from the internet layer and then makes forwarding decision with an ext information 보다 the routers at the internet layer are able come use; because that example, a gateway can use details from the web layer headers, higher layer headers, or applications data.

Hosts operate at the Application layer. A host (which could be a desktop computer PC, notebook, laptop, wireless accessibility point , or network-attached printer, amongst other devices) is generally the final destination and originator that a flow of data. A hold does no forward packets, however it maintains a routing table to facilitate forwarding decisions for packets the originate indigenous the host.

Data the flows in between layers have the right to be regarded from two various perspectives: inter-layer communication and also intra-layer communication. Inter-layer communication is the carry of details between nearby TCP/IP class on the same device and is stood for by the upright arrows in the graphic below; intra-layer communication is the transfer of information between identical TCP/IP great on different devices and also is represented by the horizontal arrows in the graphic below:


The discussions in every three parts of this are simple summaries of every of the model"s various layers. They room designed to assist you recognize what others are talking about in a conversation about networking and to assist you recognize what someone method when he or she claims that maker X operates at Layer Y, or that this difficulty is a great Z issue.

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If friend would favor to learn more about the OSI model, Cisco"s Open mechanism Interconnection Protocols web page is a good resource that gives a many of extr detail. If girlfriend would like to learn much more about the TCP/IP model, Wikipedia"s web protocol suite page, Cisco"s TCP/IP synopsis page, and Request for Comments (RFC) 1122 every contain an useful information. Another source you"ll want to know around is ours Cisco Courseware from In addition to much more complete coverage the the OSI model than both John and also I to be able to administer in these ptcouncil.nets, this courseware covers every one of the ICND1 and also ICND2 object you"ll require to grasp as girlfriend prepare for her CCNA certification.