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:
The broadcast address is:
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".
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.
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
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
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!