Learning by Doing: CISCO Certified Network ... - SCN Research

Now along the column headings we see a 1 in the columns for 64, 32, 8, 4, and 1. So we
add these numbers together 64+32+8+4+1=109. Therefore the binary number 01101101
is equivalent to the decimal number 109. Let’s do another one…convert 10010101 to
decimal.
2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0
128 64 32 16 8 4 2 1
1 0 0 1 0 1 0 1
128 + 16+ 4 + 1=149.
It’s another one of those things: easy when you know how. Let’s take a quick time out
and let you try some binary to decimal conversions:
1. 10101010
2. 01010101
3. 11001100
4. 11000101
5. 11111111
Now let’s check your answers with the answer section. Did you get the right ones? I
certainly hope so. Try not to use a calculator. You will not be allowed to use one on
the CCNA test so get practice without it now.
Converting from Decimal to Binary:
This is just the opposite of what we just did except we use subtraction. If we are given
the decimal number 141 to convert to binary we just subtract our number (141) from each
column heading in succession until we have a remainder of zero. If we encounter a
negative number then we put a zero in our bit bashing column. This is tough to explain
without working it through…so let’s learn by doing. Starting out with our 128 column
heading: 141 - 128 = 13. So we put a “1” under the 128 heading and move to the next
column heading.
2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0
128 64 32 16 8 4 2 1
1
Our next one: 13 - 64 = -51. Since this is negative we put a zero in the column heading
for 64 and move on to the next one.
2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0
128 64 32 16 8 4 2 1
1 0
50