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  1. It might be useful to see the OSI model in action. Here’s an example of telnet where the client connects to the server:

    Wireshark capture: telnet client server

    Here’s a packet from the client ( to the server (

  2. Durga,
    The Internet would break :slight_smile:

    Seriously, it would, and here’s why: Let’s say we tried to use MAC addresses as a layer 3 identity for communicating between devices at a distance (not on the same local network). MAC addresses are burned into the network cards by the manufacturer, and each card has a unique MAC (in theory). The problem comes when you are trying to keep track of who is where. The reason that the Internet is able to function now is primary due to a protocol called BGP that figures out the best way to get from one IP address to another.

    Even though there are millions and millions of connected devices, BGP is able to function by using groups of addresses–most BGP providers will reject routes for any network smaller than a /24. Being able to group large numbers of IP addresses together and treat them the same is critical so the memory and processing power required to run BGP wouldn’t overwhelm a router’s resources. In the case of try to track individual addresses on a per machine level (like we are thinking about doing with MAC addresses), this would force BGP to deal with the equivalent of /32 routes. Since devices can easily move from one location to another, there just isn’t a way to “lump” groups of them together. There would just be too many objects to keep track of within BGP!

  3. Hi Sreejith,

    I would say SONET/SDH belongs to layer 1. It specifies the physical layer and you can run different L2 protocols (including Ethernet) on top of it. It’s not like frame-relay which has a clear specification of L1 + L2.

    The main difference is that the physical layer is different between SONET on one end and Ethernet on a LAN on the other side. When you receive something from the SONET side, it goes from the physical layer to the data link layer and you end up with an Ethernet frame. The outgoing interface is selected, the frame goes from the data link layer to the physical layer and is then transmitted.


  4. When I learned the OSI Model years ago. I’ve always like this saying.

    Please, Don’t, Never, Throw, Sausage, Pizza, Away

  5. Hello Iynkaran

    First of all, the application layer is not where the actual applications on your computer function. These software applications sit on top of the OSI model and are not actually part of it. The application layer is the layer where protocols such as HTTP/HTTPS, FTP, SMTP, IMAP and others function. These protocols are then leveraged by software applications. This is what makes software applications “network aware” if you will.

    From a practical standpoint, the model that is primarily used in today’s networks is the TCP/IP model. This model incorporates parts of the session and presentation layers into the transport and application layers resulting in a model with fewer layers.

    So in your example of a web page, the web browser would use the HTTP protocol (Application layer) to communicate between the client (web browser) and the server (Web server). HTTP contains within its mechanisms the functionality of the presentation layer, so we don’t actually see the presentation layer in the Wireshark packet capture. The presentation functionalities essentially allow the information that is received from lower layers to be presented in a manner that the HTTP protocol, and the client can understand and display.

    Similarly, the session layer functionalities are incorporated into the transport layer. The sessions that are being referred to here are those between the host and server, that is between the web browser and the web server and do not involve the sessions of any other hosts.

    For more information about the TCP/IP model as compared to the OSI model, take a look at this lesson:

    I hope this has been helpful!


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