Aug 5th, 2007

if i have this wildcard mask:

0.0.0.15

does it mean that the "do-care" bits are the last 15 bits? so in binary its:

10000000.00000000?

and how i interpret this wildcard mask:

0.0.15.255

?

Overall Rating: 4 (3 ratings)

## Replies

GillieLucent Sun, 08/05/2007 - 09:38

Hi,

0.0.0.15 doesn't mean last 15 bits, here 15 means decimal value 15 for last octet.

for 0.0.15.255, binary is below :

00000000.00000000.00001111.11111111

if you want to know how 00001111 becomes 15, here comes

0 x 128

0 x 64

0 x 32

0 x 16

1 x 8

1 x 4

1 x 2

1 x 1

---------

15

-------

Only in subnet mask you can represent in bits as like below :

130.100.200.10/24 and

130.100.200.10 255.255.255.0 are same.

If you are not clear feel free to ask more.

Thanks,

Vijaybabu

milkdroogy Sun, 08/05/2007 - 09:50

well ok so 15 means decimal value - but what does it represent? what does this 15 mean?

royalblues Sun, 08/05/2007 - 10:06
• Green, 3000 points or more

As said in the revious post 15 is the decimal value of the octet

for eg. 0.0.0.15

This means the last octet is having a decimal value of 15 i.e 00001111 or (0F in hex). When using wildcard masks the 1's bit are dont care bits.

hence 172.16.100.0 0.0.0.15 would match all address between 172.16.100.1 - 14

Similary 0.0.15.255 would mean the last 12 bits are dont care bit.

10.10.0.0 0.0.15.255 would match all addresses from 10.10.0.1 - 10.10.15.254

HTH, rate if it does

Narayan

GillieLucent Sun, 08/05/2007 - 10:11

IP address is 4 octets.In every octet there are 8 bits, when converting an octet into decimal value, each bit is associated some value as below :

1st bit ( from left) - 128

2nd -64

3rd -32

4th -16

5th -8

6th -4

7th -2

8th -1

so for 00001111

0 x 128

0 x 64

0 x 32

0 x 16

1 x 8

1 x 4

1 x 2

1 x 1

---------

15

-------

Hope this helps.

Pls rate the posts.

Thanks,

Vijaybabu

milkdroogy Sun, 08/05/2007 - 11:13

if you got into binary then i have a question:

i know that

10000

-

01111

=

1

but how do i do that on paper?

i mean -

1-1 = 0

0-0 = 0

1-0 = 0

0-1 = 1(?)

how do i do that?

royalblues Sun, 08/05/2007 - 13:03
• Green, 3000 points or more

Well it goes something like this

since you cannot subtract 1 from 0 you borrow the digits ..something similar what you do with decimals

say for eg you need to subtract 28 from 52

i.e 52-28 (here you borrow one from 5 which makes it 4 and the carried digit with 2 becomes 12 and so 12-8=4 & 4-2=2 i.e 52-28=24

Similarly to do subtraction in binary 10000-01111 you will borrow the digit

1 - borrowed over & hence becomes 0

0 - becomes 10(i.e 2) again 1 gets borrowed over leaving 10-1 = 1 (ie 2-1=1)

this continues till you reach the last digit

where you have 10-1=1 and rests cancel out.

There is another method where you use the 2's component to achieve the same

10000 - 01111

complement 01111 -- becomes 10000 (just

reverse the digits)

add one to get the 2's complement

10000+1=10001

Add this to 10000 i.e 10000+10001=100001

Remove the higher order digit to get 00001

You got me back to engg days :-)

Easy method now would be to use calculator

Narayan

milkdroogy Mon, 08/06/2007 - 01:41

Thanks man!

its just that I'm studying now for my CCNA-ICND exam, that's why i have all this silly questions

:-)