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Subnetting Help

Member Posts: 12 ■□□□□□□□□□
Im trying to understand subnetting, and I end up just getting confused as hell and i have a test on this next week and i can't solve the darn thing. I have an IP address 172.16.88.4/21my submask would be 255.255.11111000.0decimal form=248. 255.255.248.0n=8(my increment bits)So now I have to find 5 things. a) subnet IDb)First Usable addressc)Last Usable addressD) Broadcast addressE) total number of host per subnet( I already know how to solve, the method my professor showed me was just to count the remaining zeroes in the submask binary format and use that as the power of M, 2^m-2=2046 in this case)But I get confused as all hell on the rest. Any help I can get, I'll be grateful. I need to learn how to do this by head, so no calculator for me.

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Member Posts: 189
TheGoon wrote: »
Im trying to understand subnetting, and I end up just getting confused as hell and i have a test on this next week and i can't solve the darn thing. I have an IP address 172.16.88.4/21my submask would be 255.255.11111000.0decimal form=248. 255.255.248.0n=8(my increment bits)So now I have to find 5 things. a) subnet IDb)First Usable addressc)Last Usable addressD) Broadcast addressE) total number of host per subnet( I already know how to solve, the method my professor showed me was just to count the remaining zeroes in the submask binary format and use that as the power of M, 2^m-2=2046 in this case)But I get confused as all hell on the rest. Any help I can get, I'll be grateful. I need to learn how to do this by head, so no calculator for me.

First, If the mask is neither 255 or 0, refeer to the OCTET AS "The Interesting Octet". Calculate the magic number as 256-mask.. Set the subnet ID value to the multiple of the magic number that is closest to the IP address without going over.....For example...

IP address: 172.16.88.4 Mask: 255.255.248.0

Magic number is = 256-248 (Interesting Octet) = 8

The multiples of the Magic numbers are 0,8,16,24.32.40,48,56,64,72,80,88<----

88 is the closest number without going over 88 ( interesting octet)

So, the subnet ID is 172.16.88.0

Broadcast ID:

--If the mask octet = 255, copy the subnet ID
--If the mask octet = 0, write a 255
--If the mask is neither, identify this octet as the Interesting Octet:

A. Calculate the magic number as 256 - mask
B. Take the subnet ID's value, add the magic number, and subtract 1... ( ID + magic - 1 )

IP address: 172.16.88.4 Mask: 255.255.248.0

Subnet ID : 172.16.88.0

Broadcast ID : 172.16.95.255

Octet 1,2,4 are uninteresting octets. So just copy the first 2 ocets of the Subnet ID in the first 2 octets of the Broadcast ID. The 4th octet is 0; so just put 255 in the forth octet of the broadcast ID. Now, the interesting octet is the 3rd octet. Use the math now. Subnet ID( 88 )+Magic number ( 8 ) - 1 =95... AND That's gonna be in the 3rd Octet of your broadcast ID & It's going to be 172.16.95.255

Networks:

172.16.88.0 ; Ip address range is 172.16.89.0 - 172.16.95.255

172.16.96.0 ; Ip address range is 172.16.97.0 - 172.16.103.255

172.16.104.0 ; Ip address range is 172.16.105.0 - 172.16.111.255

I hope this Helps & good luck to you my friend on your test. You will Pass. Just have faith on yourself.
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Member Posts: 8 ■□□□□□□□□□
If worse comes to worse you can do what many CCNA folks do and memorize a ********** and then once your test starts, write down the ********** and use that for the test. I might help.
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Member Posts: 1,772 ■■■■■■■■□□
spend an hour on this site. After you get a few wrong the answers start to make sense.

Good Luck!

subnetting.net - Subnet Questions and Answers
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Member Posts: 245 ■■■□□□□□□□
Is subnetting on the Network+ now?!

Oh boy! OP, www.subnetting.net is a great site. I use the 256 method - subtract your interesting octet from 256 and that is your increment. Then I just go from there. Practice practice practice is all I can say. I hate math with a passion, but once you get it down it's really not that bad.
A+, N+, S+, CCNA:RS, CCNA:Sec

"In high society TCP is more welcome than UDP. At least it knows a proper handshake" - Ben Franklin

2019 Goals: CCNP:RS & relocate to St. Pete, FL!
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Banned Posts: 2,496
I use the 256 minus interesting number too and then to double check my work since I'm still drilling it into my brain I used the 2nd power formula too.

Your only going to master it once you practise, practise, practise and then get drunk and then practice some more while your drunk... if you can't drink well then that's a shame, when your reasoning is out the window you seem to do better, at-least for me then again I'm also like this guy too! ... I blame the ADD!

I'm like subnetting, subnetting, ooo look a kool website, subnetting, pretty cool video game appears on phone/PC, completely lost focus now....
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Member Posts: 245 ■■■□□□□□□□
@Deathmage, coffee is your friend in this case
A+, N+, S+, CCNA:RS, CCNA:Sec

"In high society TCP is more welcome than UDP. At least it knows a proper handshake" - Ben Franklin

2019 Goals: CCNP:RS & relocate to St. Pete, FL!
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Member Posts: 15 ■■■□□□□□□□
I just took and passed this test with an 827. Boy oh boy I found it really difficult. I actually found the Linux+ exams easier. I do not enjoy how wide the area of knowledge is with zero depth. At least the subnetting had some depth and I enjoy practicing the tests on the supplied website from above. As soon as I started the exam I wrote out the following tables. 1. Bits,blocksizes/exp, decimal mask. Routing name, type. Ethernet crossover cable pin-out. TCPIP/OSI layers and device locations. Then when the exam started I read through all the simulation questions. Answered the ones I could do right off the bat. The ones I was not 100% sure on I marked down what it was and then did the rest of the exam looking out similar questions that I could mark and then go back and finish the simulations.

I found writing out the tables of information really helped with getting into the flow of the exam. I had great reference and when it came to any wireless or port questions I just quickly wrote up another table with the corresponding information and added to it as the exam went on. So all in all I ended up with just a sheet of tables.
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Banned Posts: 2,496
If your still stumped, this guy in this youtube video explains it pretty well and it's helped me with the relation of 255 to 1 and 128 to 2 and 64 to 3, yadda yadda, yadda....

https://www.youtube.com/watch?v=GSX1GlaznKM
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