Cisco Certification - The Joy Of Hex
Cisco certification candidates, particularly CCNA candidates, must master binary
math. This includes basic conversions, such as binary-to-decimal and decimal-to-
binary, as well as more advanced scenarios involving subnetting and VLSM.
There's another conversion that might rear its ugly head on your Cisco exam,
though, and that involves hexadecimal numbering.
Newcomers to hexadecimal numbering are often confused as to how a letter of the
alphabet can possibly represent a number. Worse, they may be intimidated – after
all, there must be some incredibly complicated formula involved with representing
the decimal 11 with the letter "b", right?
The numbering system we use every day, decimal, concerns itself with units of
ten. Although we rarely stop to think of it this way, if you read a decimal
number from right to left, the number indicates how many units of one, ten, and
one hundred we have. That is, the number "15" is five units of one and one unit
of ten. The number "289" is nine units of one, eight units of ten, and two units
of one hundred. Simple enough!
Hex numbers are read much the same way, except the units here are units of 16.
The number "15" in hex is read as having five units of one and one unit of
sixteen. The number "289" in hex is nine units of one, eight units of sixteen,
and two units of 256 (16 x 16).
Since hex uses units of sixteen, how can we possibly represent a value of 10,
11, 12, 13, 14, or 15? We do so with letters. The decimal "10" is represented in
hex with the letter "a"; the decimal 11 with "b"; the decimal "12" with "c",
"13" with "d", "14" with "e", and finally, "15" with "f". (CCNA candidates will
remember that a MAC address of "ffff.ffff.ffff" is a Layer 2 broadcast.)
Practice Your Conversions For Exam Success
Now that you know where the letters fall into place in the hexadecimal numbering
world, you'll have little trouble converting hex to decimal and decimal to hex –
if you practice.
How would you convert the decimal 27 to hex? You can see that there is one unit
of 16 in this decimal; that leaves 11 units of one. This is represented in hex
with "1b" – one unit of sixteen, 11 units of one.
Converting the decimal 322 to hex is no problem. There is one unit of 256; that
leaves 66. There are four units of 16 in 66; that leaves 2, or two units of one.
The hex equivalent of the decimal 322 is the hex figure 142 – one unit of 256,
four units of 32, and 2 units of 2.
Hex-to-decimal conversions are even simpler. Given the hex number 144, what is
the decimal equivalent? We have one unit of 256, four units of 16, and four
units of 4. This gives us the decimal figure 324.
What about the hex figure c2? We now know that the letter "c" represents the
decimal number "12". This means we have 12 units of 16, and two units of 2. This
gives us the decimal figure 194.
Tips For Exam Day
Practice your binary and hexadecimal conversions over and over again before you
take your CCNA exams. Binary math questions come in many different forms; make
sure you have practiced all of them before exam day. The number one reason CCNA
candidates fail their exam is that they're not prepared for the different types
of binary math questions they're going to be asked, and that they aren't ready
for hexadecimal questions at all.
You don't have time to learn how to do in on exam day. You've got to be ready
before you go into the exam room, and the only way to be ready is a lot of practice.
Finally, make sure you read the question carefully. You've got hex, decimal, and
binary numbers to concern yourself with on your CCNA exams. Make sure you give
Cisco the answer in the format they're looking for.
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free
The Ultimate CCNA Study Package, and Ultimate CCNP
Study Packages. For a FREE copy of his latest e-books, "How To Pass The CCNA"
and "How To Pass The CCNP", visit the website and download your free copies. You
can also get FREE CCNA and CCNP exam questions every day! Pass the
with The Bryant Advantage!