| Author |
Subnet question and why?
|
|
| MaulerJ 2003-01-04, 12:39 pm |
| If I have an IP address of 192.65.10.0 /28 how can I find how many subnets, and host are available.... can someone walk me through it | |
| mitchflorida 2003-01-04, 1:04 pm |
| quote:
Originally posted by MaulerJ
If I have an IP address of 192.65.10.0 /28 how can I find how many subnets, and host are available.... can someone walk me through it
14 Subnets with 14 hosts apiece
2X2X2X2 - 2+= 14 | |
| MaulerJ 2003-01-04, 2:23 pm |
|  awesome I guess, but how, 2*2*2* etc? where and why do you get those numbers, For my Network plus I practiced a bit of subnetting, but it never went in deep, oit was just basic, what is the range , and that was it, I think I am easily confused with this area. even after reading the FREAKS subnet page. | |
|
| hey j try this site if you haven't been there yet go to ccna section and there are some subnetting guides,etc they helped me out alot.good luck www.ciscotrack.com | |
| larkspur 2003-01-04, 7:45 pm |
| Mitch,
I maybe mistaken but I believe that subnet is wrong. 2x2x2x2=16-2=14. I belive the number was 28. If so would you not need the extra bit. 2x2x2x2x2=32-2=30. That still leaves us 2 bits more than we need and puts us in a better situation with ip addesses for the network. Please correct me if I am wrong and show me what I missed. | |
| mitchflorida 2003-01-04, 10:52 pm |
| please read up on CIDR. . . 28 is the CIDR number , which is equivalent to a 255.255.255.240 subnet mask | |
| larkspur 2003-01-04, 11:07 pm |
| gotcha thanks for the input. 256-240-16-2=14.
Should have seen that. Thanks again. | |
|
|
| MaulerJ 2003-01-05, 11:25 am |
| Guess I am expecting an explanation, not just an answer | |
| ChrisDfer 2003-01-05, 12:41 pm |
| MaulerJ, this is the way I do I mean I'm sure there is a better but I find doing it this way is easier for me. First i figure out what the mask would be in binary. In this case its 11111111.11111111.11111111.11110000 I then count the number of host bits which is four(count the zeros) Then use the formula 2^4-2 which is 14. Then when figuring out the subnets I go 2^4-2 because I have 4 network bits in the mask. Now if I understood this next part correctly I have to subract two from the results unless all the devices in the network support SUBNET ZERO which allows the subnets to be made of 1's and 0's. Ehnnn, but then again I am probably wrong here so just ignore me. | |
| asteheske 2003-01-05, 12:45 pm |
| Hi there.
your ip is 192.65.10.0/28
it is a class c ip which means it has
network.network.network.host
since each network is 8 bits thus all the network add together will be 24 and you borrowed 4 bits from the host thus making it 28(/28)
so from there...you take 2^x where x is the number of bits you borrowed in this case is 4.
so 2^4=2x2x2x2=16
now you got 16 subnets...but you still haven't take into consideration of the subnets that cannot be used.(ie network & broadcast)
thus you subtract 16-2=14
and voala!....you got your answer...14 useable subnets with 14 host per subnet.
I hope these helps....cheers.....if there is sumthing wrong..on my above xplaination...pls correct me...we are afterall learning here....right??
| |
| babu75 2003-01-05, 3:35 pm |
| to add on this, its a prefix length subnetting (28).the exact sunets (14) are 16,32,48,......224. the 1st and last subnets which are 255.255.255.0 and 255.255.255.240 identify the class c mask and the subnet mask itself respectively and cannot be used.
each of the above subnets i.e subnet(16) has host-range(17-30)and broadcast address(31,which is just before the next subnet).
i hope this clarifies further on what was posted.
happy new year to all!!! | |
| MaulerJ 2003-01-06, 9:36 am |
| Not too bad, I thought I was the only one having problems, looks like some other folks, have the concept, but still rough around the edges as well. | |
| cybertechno 2003-01-06, 11:12 am |
| Hi all,
I got a question like this on my test day.and unfortunately I missed it.But the question required one to find the broadcast address.tell me how would u find a broadcast address for such an Ip address?!?!?! | |
| MaulerJ 2003-01-06, 12:31 pm |
| My understanding, with help was that this would be a bad IP address to explain this with, because the .0 would be the network address, so please put up another address, and now that i am getting some practice, will send back an answer | |
| ANDRONDA 2003-01-06, 5:42 pm |
| You guys have to get over this policy of subtracting out the all 0's and all 1's subnets when you do this.
Cisco does not follow that. On a /28 subnet mask they would say that you would have 16 subnets and 14 hosts per subnet. If Cisco asks how many subnets and hosts per subnets would you get with any class C IP address and a subnet mask of 255.255.255.240 and you answer 14 and 14 YOU WILL MISS THE QUESTION.
You do not subtract two out of the subnets but you MUST subtract out the two host addresses that are the broadcast and network address.
I am not trying to confuse you or mislead you. I scored very high on the CCNA test. Ask anyone who has taken the test recently.
As for why it is simple. They identified two things you need: the IP address falls within Class C range and you have a /28 mask. 95% of the work is done.
First put down the default mask for class C: 11111111.11111111.11111111.00000000
This is 24 bits total (count the 1’s up). Add four more 1’s make it 28. You should have11111111.111111111.111111111.11110000 or 255.255.255.240.
Count up all your borrowed 1’s (the four you added) and raise the number 2 to that power: 2 to the 4th power is 16. Then count up the remaining zeros and raise the number 2 to that power. This will be the number of hosts. There are four. So 2 to the 4th power is 16 again. Then you subtract out the two because you cannot use the broadcast or network IP addresses. This is how you get 14 hosts. | |
| mexican 2003-01-06, 9:51 pm |
| there might be 16 actual, but i was taught by the cisco networking academy as there are only 14 usuable, unless you use the ip subnet-zero command? i might be wrong but i dont understand why a certified networking professional would teach me wrong? im only 18 i would hate to start out wrong and have to start all over learning how to subnet again cause its hard enough as is anyone got any ideas? thanx | |
| freak 2003-01-06, 10:09 pm |
| quote:
Originally posted by JetGDI
Check out Freak's guide to subnetting.
That should be all you need to get started at least.
http://www.mcsefreak.com/subnetting.htm
Thanks you saved me a post | |
| bass2k1 2003-01-06, 11:09 pm |
| Well, I don't know about any of the rest of you, but this thread sure as hell helped me understand things a whole lot better!
Thanks to all who contributed.
Regards
sebastiaan.rothman@bhpbilliton.com | |
| asteheske 2003-01-06, 11:24 pm |
| From the way i see it...there is a couple of method or ways..for you to solve this subnetting problems(if you may call it problem)...and bass21k's is one that is quite easy to comprehend.
As for my posted thread abt this subnetting issue earlier, it is the way i learned and so i share ... but then again bass21k is much easier to understand.
I have been enlightened!
THank You~ to all.
cheers~ | |
| bass2k1 2003-01-07, 12:33 am |
| Also, another question. Could someone please explain the format of the block notation for subnetting, i.e /28 instead of 255.255.255.240.
With Class C addresses it is very trivial, but what happens when it comes to Class B or even Class A subnets?
Regards
sebastiaan.rothman@bhpbilliton.com | |
| mexican 2003-01-07, 12:39 am |
| you count the "1;s" in the network portion of the subnet mask....
255 . 255 . 255 . 240
11111111.11111111.11111111.11110000
8 8 8 4 =28
i guess thats what you were asking...? | |
| bass2k1 2003-01-07, 12:41 am |
| quote:
With Class C addresses it is very trivial, but what happens when it comes to Class B or even Class A subnets?
Regards
sebastiaan.rothman@bhpbilliton.com | |
| ANDRONDA 2003-01-07, 10:03 am |
| quote:
Originally posted by mexican
there might be 16 actual, but i was taught by the cisco networking academy as there are only 14 usuable, unless you use the ip subnet-zero command? i might be wrong but i dont understand why a certified networking professional would teach me wrong? im only 18 i would hate to start out wrong and have to start all over learning how to subnet again cause its hard enough as is anyone got any ideas? thanx
The guy is telling you what is done in the field and not necessarily on the test.
It goes against the CCNA agreement to post actual questions here but I will tell you that I had a question on the test that gave an IP address and subnet mask and then asked how many useable subnets and hosts will you get from this IP/Subnet combination.
I saw the same question on one of the test prep softwares and in the explanation it had said to assume that ALL SUBNETS are available for use unless explicitly stated in the question that they are not (because of legacy hardware/software). My CCNA question did not say anything about legacy hardware so I answered assuming that all subnets were useable.
Of course I do not have an answer key but I assume I answered correctly. I only missed a few on the test anyway ant THAT one I do not believe was one I missed.
Go ahead and ask your instructor and I suppose if you have faith in him go with what he says. Print out my post and show it to him and see what he says.
I do not want you to miss this one over a misunderstanding. | |
| MaulerJ 2003-01-07, 12:20 pm |
| Good one, i have seen some questions, saying with old UNIX, blah blah how many subnets would you have. If that is the case then for old software we use all the subnets and new software we don't? or is that vice versa | |
| Imran4sin 2003-01-08, 12:33 am |
| my 2 cents ;;>
Well, If the question asks for usable subnets then I guess answer should be 14 . But when it asks for number of subnets then shouln't it be 16.
So what would be the answer to the original question!!!
14 or 16 ????
| |
| ChrisDfer 2003-01-08, 12:48 am |
| Well if your devices support SUBNET ZERO then you would have 16 usable subnets not 14. And from my understanding the new IOS's have subnet zero enabled by default. | |
| Imran4sin 2003-01-08, 2:07 am |
| quote:
Well if your devices support SUBNET ZERO then you would have 16 usable subnets not 14.
First one is for local loop--cant be used,
Last one is for broadcasting--cant be usd.
So!!!gimme a number (14 or 16). I thnk it's 16. Since the question doesnt ask for usable. | |
| ANDRONDA 2003-01-08, 2:59 pm |
| Eureka! I just remembered.
They asked for example.
Calculate the number of useable networks and hosts for 200.106.1.0/25 :
A. 4 Networks with 62 hosts
B. 8 Networks with 30 hosts
C. 32 Networks with 6 hosts
D. 2 Networks with 126 hosts
It was always worded in such a way that you did not have to decide because the choices were so obvious. In other words they did not offer one choice with the all 0’s all 1’s AND one where you would subtract out the two. | |
| chodan 2003-01-09, 7:53 am |
| I have found that to be the case also.
They will not try to trick you contrary to popular belief.
SO you wont have to chose between 14 or 16.
In the RW you will use 16 and use the ip subnet 0 command.
Why?
becasue you don't leave a bunch of addresses out there unused.
You will probably need them. |
|
|
|