|
|
|
| I notice when I do practice test questions and subnetting is apart of the question, its always formed as an IP address /# (ex: 192.168.10.52 /30) but in Lammle's book he always gives us the subnet mask address which makes it easier to figure out the available host addresses for that particular IP address and subnet. How does the /30 let me find out what the subnet mask is? This may be an easy question for you guys but this is really baffling to me that I can't get this completely figured out. Any advice is greatly appreciated. | |
|
| here is an easy solution /30 is the number of bits.
the way I remembered it was to write the bits out in reverse.
You know that to get the ip from the bits in binary, each octet has 8 bits of ones and zeros.
128 64 32 16 8 4 2 1
if you put a one under each number and add those numbers to get your ip for each of the four octets that will give you your whole ip.
the mask is just the opposite
128 192 224 240 248 252 254 255
which is just adding each bit for each octet.
If you had a /30 bit mask you would count over 30 from left to right.
24 bits gives you class c 255.255.255.0
or 11111111.11111111.11111111.00000000
add 6 more bits to make it 30 from 24
so it would look like
11111111.11111111.11111111.11111100
use the second numbering scheme for the last octet
128 64 32 16 8 4 2 1
1 1 1 1 1 1 0 0 (add these numbers or)
128 192 224 240 248 252 254 255
1 1 1 1 1 1 0 0
the last 1 is your subnet = 254
hope this helps
| |
|
| It means that there are 30 "1's" in the subnet mask: 11111111.11111111.11111111.111111 00
255 255 255 252
Where ever you see the "/##" at the end of an address the "/##" is for the subnet mask, just fill in the corresponding fields with 1's. Then do the conversion and you will find what you are looking for.
Good luck. | |
|
| I meant 252, I fat fingered it! LOL | |
|
| OK... THE WAY I LEARNED IS THIS WAY
/30
YOU AUTOMATICALLY KNOW THAT THIS IS:
255.255.255.0<<
^ ^ ^ = 24
SO YOU ARE GOING TO BORROW 6 BITS:
128+64+32+16+8+4=252
THIS EQUALS TO:
255.255.255.252
KNOW YOU SUBTRACT:
256-252=4
KNOW THE HOSTS ARE LIKE THIS:
4,8,16,24 ETC
2(10) -2= 766 HOSTS
EX. 192.168.10.52
^
SO THE ANSWER FOR YOUR QUESTION IS IN THE SUBNET BETWEEN 8 AND 16= 9 AND 14
IF I GOT IT WRONG SOMEBODY PLEASE CORRECT IT. | |
|
| All of your responses are great...I totally understand now;
Thanks a bunch!!!!!!!!!!! | |
|
| One point on this one--might help someone.
I was taking one or another of the practice exams and they caught me, master of subnetting  with a trick question.
The question was something like you have a 22 bit subnet mask, etc. So, I figured this was 255.255.252.0
I was wrong--the explanation for the answer pointed out that if a Cisco exam says you have X bits of subnetting, that is in addition to the default subnet mask. Therefore, they pointed out, it had to be a class A addie with a default subnet of 255.0.0.0 as that was the only one that had room for 22 more bits, making it 255.0.0.0 PLUS 22 bits, making it 255.255.255.252
HTH someone
Scott | |
|
| That last note sounds nasty! So the question would have been something like: "You have a class A network with 22 bits of subnetting..."
I thought Cisco were meant to be testing our KNOWLEDGE. The whole issue of subnetting took me barely any time to learn, yet I could be tripped up on an exam by a technicality such as this. Lame, if you ask me...
Deets | |
|
| Well, in the online test, they DIDN'T mention that it's a class A addie--you have to figure that out because the 22 bits of subnetting are in addition to whatever subnet is already present.
Unfortunately, I don't remember which online test this was. I do remember, however, from doing the MCSE that many of the practice exams were actually far harder than the real MS exam.
Scott |
|
|
|