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  1. Hi Rene,

    A good understanding of ethernet for beginners like me, this helps me so much in my workplace. The explanations with real pictures of device make me clear understanding of concepts
    Thank you

  2. Hello Mohammad

    Yes, there are several types of Ethernet framing, two of which are the most common that you mention above.

    Ethernet II has a two byte EtherType field that identifies the upper layer protocol (e.g. IP) that encapsulates the frame. For example, a value of 0x0800 indicates an IPv4 packet while a value of 0x0806 indicates an ARP frame.

    The IEEE 802.3 standard changes the role of the EtherType field to a “Data Length Field”.

    Later on, because both types were in wide use, IEEE 802.3 incorporated Ethernet II and standardised both formats.

    Today, both of these formats can coexist on the same network. The method by which network devices know how to interpret each frame as one or the other is by the EtherType /Length field. If it is >= 1536 (0x0600) then it is an Ethernet II frame and that field is interpreted as an EtherType field. If it is <= 1500 it is an 802.3 frame and that field is interpreted as a Length field. Values between 1501 and 1535 are considered undefined and under normal circumstances should never be encountered. Therefore both standards can use the same medium.

    I hope this has been helpful!


  3. Hello Aditya

    Traditionally, routers, switches, and computers require a different type of Ethernet cable to connect to each other. There are crossover cables and straight through cables, as described in the lesson.

    What auto MDIX does is it automatically detects the kind of cable required for a particular connection, and if the cable used is the wrong type, then it automatically switches the appropriate pins internally on the switch or router, such that the correct connectivity is achieved. So for example, if you use a crossover cable to connect a PC to a switch (which is the incorrect cable type) the auto MDIX feature will detect this and internally switch the Tx and Rx pins so that the connectivity is achieved.

    I hope this has been helpful!


  4. Hello Adrian, good to have you with us! Glad to hear that you’re enjoying the lessons!

    In order to understand what is going on, let’s take it step by step. The packet has arrived from the Internet at the SOHO router. Now we don’t know what kind of header we’ll find at layer 2, as this depends on the technology being used. If it’s ADSL, serial link, or cable modem, each of these have a different L2 technology, and therefore a different L2 header. Whatever it is, the L2 header is stripped off, and we are at Layer 3 of the OSI model. So we take a look at the IP header. It has the outside (public) address of the router as a destination address.

    Now is you correctly stated, this IP address is translated using NAT, and the corresponding inside IP address replaces the destination address. Once the IP header is correctly translated, the next step is for the SOHO router to decide where to send the packet. In most cases, there is only one choice, as there is only a single routed port on the inside of such devices. (Remember that a SOHO router connects the WAN to the LAN using layer 3 routing).

    So the router then has to add a L2 header with the appropriate MAC address. In order to do so, it takes a look at its ARP table to see if this destination address (the newly translated internal address) exists in its ARP table. If it does, the destination MAC is populated, the source MAC is it’s own, and the frame is sent. If it doesn’t exist in the ARP table, then it sends out an ARP request for the particular destination IP, receives the destination MAC address, populates the appropriate field in the L2 header and sends it on its way.

    I hope this has been helpful!


  5. Hello Adrian

    Your explanation is correct, that’s how it works. Now jut to clarify, the Layer 2 technology used to reach your home/office will determine what exists at Layer 2. But that exists only for the portion of the transmission between your ISP and your premises. However, layer 3 remains unchanged throughout the whole transmission from end to end (except of course where NAT is involved). This is the beauty of the OSI layered model, you can change Layer 2 technologies without touching the functionality of Layer 3 and above.

    I’m glad this has been helpful!


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