Subnetting WTF.



  • Quite a while ago on the Something Awful forums somebody asked for help with subnetting. They were reading in a book for their class and wanted to know why it was so hard to understand.



    Let's get the giggles out about running out of IP addresses in 2005, and the fact that they put in a URL that they don't control which no longer points to any sort of news (spam site?). Now that you don't have the giggles take a look at the table. This table is supposed to tell you how to determine what it he network and host portion of an IP address. ANDing had not been discussed yet, and even if it had the paragraph is very clear that this table will explain all of the reader's questisons. Let's go over what's wrong here.

    1. We are not "calculating an IP address with a subnet mask", we are
    calculating the network address for an IP address with a subnet mask.
    2. The table explains nothing.
    3. Those are not "address components".
    4. That's not a valid subnet mask; you don't get to skip binary digits
    in a mask. If you want 68 hosts per subnet the correct mask is 255.255.255.128
    5. The last row in the first cell should say "Number of hosts addresses",
    but even then it's still wrong. Hosts go from the network address to
    the broadcast address, not some arbitary point in the middle.

    I wonder why he found it to be so confusing!



  • Actually, that mask is technically useable.  However, it would make anyone trying to manage that network go berserk in a few hours.  I don't know of a DHCP server that will give out a discontiguous block of addresses, so that would make network management even harder.

    BTW, that network has 14 hosts, the addresses are:

    146.118.105.136
    146.118.105.137
    146.118.105.138
    146.118.105.139
    146.118.105.140
    146.118.105.141
    146.118.105.142
    146.118.105.143
    146.118.105.201
    146.118.105.202
    146.118.105.203
    146.118.105.204
    146.118.105.205
    146.118.105.206

    The broadcast address is:

    146.118.105.207

    Every other criticism you make is valid and I agree that it is the lamest attempt ever at explaining subnetting.  Who edits these books?  I like where they say "Table 1-2 indicates how to determine it" yet Table 1-2 contains no clues whatsoever at how to determine the values.  Books like this actually make people dumber.  I've seen student on the cusp of "getting it" read a book or web page like this and suddenly "lose it".



  • Poor sod.

    I too remember being in this position. Books weren't cutting it and served only to confuse me further, and make it all seem more mysterious than ever.
    Then a ray of light- an acquaintance who worked in a NOC heard of my plea, sat down
    with me for 15 minutes, aided by no more than a pen and a piece of paper.

    That was it. Clouds lifted. Sunshine poured through. And what I couldn't believe was how easy it all was.

    Books suck.



  • It's stupid books like this that cause WTFs for people down the line
    when they get a job. I'm sure we've all met so called "network admins"
    who have probably learned their entire networking knowledge from this
    table.



    Subnetting should be taught in binary, unless your target audience can
    do implicit decimal->binary->decimal conversions in their head.
    Teaching subnetting in decimal is like teaching German grammar in
    Chinese.




  • actually...

    the netmask chosen is indeed stupid. Insane even. A shooting offense.

    The host address, however, is correct (and correctly labelled).

    Look at the last octets:

    IP is 204 = 11001100

    subnet is 184 = 10111000

    network is (ip and subnet) = 10001000 = 136

    host is (ip and not subnet) = 01000100 = 68



  •  you are so right, a good teacher makes all the difference.  I still remember my High School Spanish teacher, Mrs. Brady, sitting down with me for about 20 minutes after class when she saw I wasn't quite "getting it" and it had the same effect.

    I wish my High School math teachers were so good!


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