• Welcome to Stump the Chump!

    The above graphic depicts a simple network with one AP, one Wi-Fi STA, a L2 switch and a wired node (the server). For the sake of simplicity, let???¡é?¡é?????¡é???¡és say that the wireless STA (ABC) sends a ping packet destined for the server (JKL). This ping request travels via links 1, 2 and 3. Now, imagine a ping reply coming back from the server to the wireless STA via links 4, 5 and 6.

    Your challenge is to fill out the table with the proper MAC addresses as they would be in the header of the 802.11 or 802.3 frame. Of course, 802.3 Ethernet frames only have two MAC addresses. However, Wi-Fi can have up to four MAC addresses per frame. I???¡é?¡é?????¡é???¡éve left you space for all four MAC addresses if they are necessary. Make sure you put the addresses in proper order and if you get stuck, both the 802.3 and 802.11 standards specify the order of the MAC addresses.

    Good luck!

    Please only post the answers, not any explanations at this time. I'd like to give everyone a chance to post their answers. Thanks!

    Here is the format I'd like you to use when responding. Just copy, paste and edit for each link.

    Link #1
    Address 1:
    Address 2:
    Address 3:
    Address 4:

    GT Hill
    CWNE #21

  • OK here goes... I guess I'll be the first embarrass myself. This is off the top my head.

    Link #1
    Address 1:DEF RA/BSSID
    Address 2:ABC SA/TA
    Address 3:JKL DA
    Address 4: Unused

    Link #2
    Address 1:JKL DA
    Address 2:ABC SA

    Link #3
    Address 1:JKL DA
    Address 2:ABC SA

    Link #4
    Address 1:ABC DA
    Address 2:JKL SA

    Link #5
    Address 1:ABC DA
    Address 2:JKL SA

    Link #6
    Address 1:ABC RA/DA
    Address 2:DEF TA/BSSID
    Address 3:JKL SA
    Address 4: Unused

    Thanks for the little exercise for my brain this evening.

  • Keith did get the right answer which is surprising for a CWNE... :)

    I present this question in class to my students and surprisingly enough, rarely does someone get it right. There are two keys to getting the answer correct for every addressing question. First, a few definitions:

    Receiver: This is the MAC address of the wireless device that will be receiving the frame. This is always the first address in a Wi-Fi MAC header. Remember, the receiver isn't always the final destination.

    Transmitter: This is the wireless device that is transmitting the frame. This is always the second address in the frame. Remember, the transmitter isn't always the source.

    Key #1: An AP is a L2 device that just passes traffic like a switch. It does have an extra special function of "converting" 802.11 frames to 802.3 and vice-versa.

    Key #2: You should always be able to "follow" the address. You should be able to go from link 1 to 2 to 3 to 4 and never wonder how the data would get from one place to another.

    Key #3: Addresses in a Wi-Fi MAC header are "as needed". You will see in the dialog below that sometimes the source and transmitter are the same, so there is no reason to list both.

    Link 1: Notice that there is no source specified. That is because when a STA is transmitting to an AP, the STA is always the source, so it is implied and does not exist in the header because it would be a waste of space.

    Link 2: This of course is just Ethernet, but you may be tempted to use GHI as the source. An AP is a Layer 2 device and processes MAC addresses just like a switch. It will never change the MAC header, just forward the frame based on the contents.

    Link 3: I just added this to make it a bit more complex, but a switch doesn't change MAC source and destination information.

    Link 4: The destination is the STA, not the AP. Even though the AP has a MAC address of its own on the wired side, no data (except management traffic) will be destined for the AP by MAC.

    Link 5: See link #3.

    Link 6: Notice that the Receiver and Destination are the same thing. This is because when an AP is transmitting to a STA there is never a time when the receiver address is different than the final destination. They are always the same, so there is no reason to waste the space the header.

    If you have any questions on MAC addressing, feel free to post here and we'll do our best to answer the questions.

    GT Hill

  • Thanks for posting the question! It is a very important question with many pertinent points all in WiFi should understand.

    This shouldn't have been a 'stump the chump' - but a 'gimme' question for all CWTS/CWNA's out there!

    This is IMPORTANT to know for troubleshooting.

  • Hrmm, someone just gave me a new interview question.



  • You are welcome! I think you'll find that not may can answer the question, but if they can, they have something going for them!


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