The knowledge about Host-to-Host communication in a network is essential for a person to pass CCNA 100-101 exam as it is one of the major sections in the course outline.

Understanding of Host-to-Host Communication Model:

Suppose we have to laptops as shown in the figure below and we want to share information between these laptops. For this purpose we have to connect them via Ethernet or wireless but here we’ll assume that we have connected with Ethernet cable. While transferring information from one laptop to other, some reference models are being followed that can be OSI Model or TCP/IP Model.


Proprietary Infrastructure:
In the old times there were no rules defined for the communication.  If you have two machines by different manufacturers i.e. IBM and Dell, you cannot connect them for data communication. At that time each device in a network i.e. routers and switches, should be from the same manufacturer to enable the communication.

So the industry standard body comes up with the idea to overcome this issue of proprietary and introduced the standard based model so that the communication between the devices should become manufacturer independent. This was made possible because all the manufacturers had to follow the standards defined by the standard body. All the manufacturers follow OSI or TCP/IP Models.

OSI Model:
OSI model stands for Open Systems Interconnection model. OSI model is seven layer model as shown in the figure below.


All these layers perform the specific tasks so that that communication between two devices can be done smoothly. Moreover each layer helps the other one in its specific task

  • Physical Layer:
    The physical layer in the OSI model deals with the physical connection between two interconnecting devices. The physical layer basically defines all the electrical and mechanical rules. The message is sent through the electrical pulses. Moreover the physical layer also defines the layout of the network devices or their topology. The transmission mode of the communication is also defined in the physical layer whether it is simplex, duplex or full duplex.
  • Data Link Layer:
    The data link layer basically defines the format of data being transmitted across a network. Other features of the data link layer include Error Detection and Error Correction. The data link layer deals with the physical address of the devices. The data link layer is further divided into two sub layers

MAC Layer: MAC stands for Media Access Control. This layer defines how the devices will be permitted to access and transmit data.
LLC Layer: LLC stands for Logical Link Control. This layer is responsible for error checking and data packets synchronization.

  • Network Layer:
    It is the one of the most important layers in this model and routing takes place in it. This layer provides connectivity and path selection. Because of connectivity and path selection we can communicate between separate networks. The separate networks mean that the networks are far away from each other geographically. The network layer deals with logical or IP addresses.
  • Transport Layer:
    The transport layer provides services used to segment, transfer and reassemble data. The transport layer also determines how we send our information.
  • Session Layer:
    The session layer creates, manages and terminates the sessions. Suppose two users are connecting to a same server and requesting for two different images. Now the server has to distinguish between these two users and their requirements. The session layer helps the server in this regard.
  • Presentation Layer:
    Presentation layer has the one job that the information sent to the application layer of one system is readable by the application of the other. In other words the image shown on one system should be also shown properly on the other system.
  • Application Layer:
    The application layer is closest to the end user. This layer interacts with the software application     to display the information to the end user.

TCP/IP Protocol Suite:
TCP stands for Transmission Control Protocol. The former name of this model was DOD Model. Here DOD stands for Department of Defense model because the devices developed for the military were based on this model. The TCP/IP model is composed of following four layers as shown below


If we compare this model to OSI model, the physical and data layers are joined to form link layer in TCP/IP model. Application, presentation and sessions layers from OSI model are joined to form the single application layer in TCP/IP model.

  • Link Layer:
    The link layer in TCP/IP model is responsible for the physical connections and how things are accessed and controlled. The link layer also control the transmission of data packets from one system to another.
  • Internet Layer:
    The internet layer is responsible for the routing of data, which means moving information between different networks. The internet layer makes sure that the devices which are on different networks are able to communicate with each other. The internet layer moves data from link layer to the transport layer or we can say that it acts as a bridge between link and transport layer.
  • Transport Layer:
    The transport layer provides communication services. The session information takes place in this layer. Session perform the same tasks as discussed earlier in OSI model.
  • Application Layer:
    The application layer includes APIs, browsers, email clients and other application that are responsible for the presentation of data to the end user.


  • The OSI Model is composed of seven layers but TCP/IP model has four layers.
  • OSI is a reference model and TCP/IP is an implementation of OSI model

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Passing Cisco 100-101 Exam:
After getting all the technical knowledge about 100-101 exam, the candidate have to practice for better preparation of the real exam. The practice tests are a best tool / way to prepare for the real exam as it is a simulator of a real exam.

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