# Subnet Mask, Decimal, Slash Notation.

Discussion in 'Server Networking' started by Robert Bollinger, Oct 6, 2007.

1. ### Robert BollingerGuest

Hey Guys,

I am hoping that you can assis me with this question, as i have hit a wall.
I need to have this explained to me, this is in my 70-291 book on page 2-16.

128.000.000.000 has a binary value of 10000000.00000000.00000000.00000000

This is mask to binary conversion, i can memorize this all day but would
rather understand how and WHY it works like the above example.

How come a binary notation of 10000000 = 128? how is that done? why does it
start at 128? and not at 1?

HOw are the bit's determined and converted into a dotted.decimal address?

i understand this: 32-bit ip addy is 4 groups of eight bits at different
placements (starting) with 0-8 9-16 etc.

theses are called octets which means eight or of eight. Wht i dont
understand is as stated above.

I am sorru of this is not clear but i am fried on this one.

Robert

Robert Bollinger, Oct 6, 2007

2. ### Bill GrantGuest

An octet is eight bits. In binary, each column can only have the value of
zero or one.

So binary 10 = 2 decimal
100 = 4
1000=8
10000=16
100000=32
1000000=64
10000000=128

Bill Grant, Oct 7, 2007

3. ### Robert BollingerGuest

Can you be more specific with more examples about that, i am kinda getting
it but there is a difference between "I think I know" and "I have a clue" and
"I really know, yes i can fix your network".

Robert Bollinger, Oct 7, 2007
4. ### jorgenGuest

In our normal decimal system (base 10)

1 is one
10 is ten
100 is hundred

each time you put a zero in, the value goes up by a factor of 10. In the
binary system (base 2) the value goes up by a factor of 2:

1 is one
10 is two
100 is four
....
10000000 is 128
01000000 is 64

jorgen, Oct 7, 2007