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scottsee
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Subnetting

Thu Nov 11, 2010 10:38 pm

I've been doing subnetting for the past couple weeks and it's just not totally sinking in.. I'm good at it, I know what I'm doing, but I just have a hard time with certain aspects of it. I'll be on a roll and have no trouble with it then jump back to it a day or two later and will really sit and stare at the question for a minute and dissect the wording, break out the paper and pencil and have to write it out. I feel like I'm just starring thought it, it's thought to explain. I'll have no trouble doing one type of subnetting problem one night, and then completely struggle with it a couple days later. How long has it taken most of you to commit the entire scope of subnetting to memory.

I've been using the resources here, on the Train-Signal Exam's, Online. I guess I just need to keep plucking along.. :|

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Re: Subnetting

Thu Nov 11, 2010 10:53 pm

Uhh... Been at this for 8 or 9 years now. I still break out paper and pencil regularly to subnet. I do go through phases though where if I'm subnetting over and over again it gets into memory fast and I don't always need to write it out. But then there are times when I won't have to subnet for a couple months and I'll stare at 255.255.255.248 and for the life of me can't figure out that that it's a /29 mask with 8 addresses (6 usable).

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Re: Subnetting

Thu Nov 11, 2010 10:57 pm

Happens to the best of us I think. Embarrassingly enough; I got half way through CCNP ROUTE before going "shit, I need to go back to ICND 1 and learn how to efficiently subnet again" :lol:
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Re: Subnetting

Thu Nov 11, 2010 11:19 pm

I'm in the same boat....I spend at least an hour a day if not more on the practice subnetting here on the site practicesubnetting.php Though I think I might be handicapping myself because that damn chart is there to the left =P Sure with the chart I can do the subnetting question in about 15-20seconds.....but I really need to memorize the CIDR's Once I get that, I'm golden.
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Re: Subnetting

Thu Nov 11, 2010 11:34 pm

One thing I tend to do a lot in my head is count the bits in binary. eg. 1 2 4 8 16 32 64 128. But actually, I do it backwards since this is all represented in little endian. 128 64 32 16 8 4 2 1.

I also find that I need to remember the addition as well. 128 192 224 240 248 252 254 255.

128+64=192
192+32=224
224+16=240
240+8=248
248+4=252
252+2=254
254+1=255

By memorizing those two sets of numbers I find I can calculate a given subnet fairly quickly.

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Re: Subnetting

Thu Nov 11, 2010 11:59 pm

Hey scottsee! I feel U bro! Some questions U breeze n some U 've got to get the pen/pencil to make sure u don't screw up!

I think by memorizing block sizes n mask n all valid subnets/host for the block U 'll do it much faster n better!
I like what I see in Lammle new CCNA book chapter 3 "subneting in your head."
It's working for me!

Try that too!
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Re: Subnetting

Fri Nov 12, 2010 12:23 am

Infinite wrote:One thing I tend to do a lot in my head is count the bits in binary. eg. 1 2 4 8 16 32 64 128. But actually, I do it backwards since this is all represented in little endian. 128 64 32 16 8 4 2 1.

I also find that I need to remember the addition as well. 128 192 224 240 248 252 254 255.

128+64=192
192+32=224
224+16=240
240+8=248
248+4=252
252+2=254
254+1=255

By memorizing those two sets of numbers I find I can calculate a given subnet fairly quickly.


Not bad, actually pretty helpful for doing it in my head without the chart...just take the sum, subtract it from 256 and you have your range of ips. So...

Normally when I do subnetting in my head without the "Magic Chart" I follow some crazy math logic like converting binary in my head

What are the subnet and broadcast IPs for host 10.65.231.31/22?

22 is 22 bits on 11111111.11111111.11111100.00000000
255.255.xxx.0
take the 6 bits on in the 3rd octet and add them up 128+64+32+16+8+4=252
256-252 = 4
So we know the ips are in multiples of 4
4x 60 is 240 which is too high, but close enough to know that if you take 2 away you're at 232 so make it 3 and your at 228.0 which is your subnet ip address and that makes 231.255 the broadcast


However doing it that way, I know the last bit on is the 6th so 248+4 = 252
256-252 = 4

Way less steps and less steps = less time, I'm down with that...thanks :)
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Re: Subnetting

Fri Nov 12, 2010 1:12 am

Well, you've taken a different approach that me, but if it works for you then give er shit. :)

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Re: Subnetting

Fri Nov 12, 2010 2:27 am

Axis wrote:
Infinite wrote:One thing I tend to do a lot in my head is count the bits in binary. eg. 1 2 4 8 16 32 64 128. But actually, I do it backwards since this is all represented in little endian. 128 64 32 16 8 4 2 1.

I also find that I need to remember the addition as well. 128 192 224 240 248 252 254 255.

128+64=192
192+32=224
224+16=240
240+8=248
248+4=252
252+2=254
254+1=255

By memorizing those two sets of numbers I find I can calculate a given subnet fairly quickly.


Not bad, actually pretty helpful for doing it in my head without the chart...just take the sum, subtract it from 256 and you have your range of ips. So...

Normally when I do subnetting in my head without the "Magic Chart" I follow some crazy math logic like converting binary in my head

What are the subnet and broadcast IPs for host 10.65.231.31/22?

22 is 22 bits on 11111111.11111111.11111100.00000000
255.255.xxx.0
take the 6 bits on in the 3rd octet and add them up 128+64+32+16+8+4=252
256-252 = 4
So we know the ips are in multiples of 4
4x 60 is 240 which is too high, but close enough to know that if you take 2 away you're at 232 so make it 3 and your at 228.0 which is your subnet ip address and that makes 231.255 the broadcast


However doing it that way, I know the last bit on is the 6th so 248+4 = 252
256-252 = 4

Way less steps and less steps = less time, I'm down with that...thanks :)


That's nuts. :shock:

I just take the IP 10.65.231.31/22 and think in terms of /8, /16. /24 where is it? /22 is two binary bits into the 3rd octet from the right, which makes the Subnet increment a multiple of 4, then just figure out the closest common multiple of 4 and 231, which 228. making 228.0 the network, 231.255 the broadcast and everything in the middle fair game.

What I have the most trouble with are story type question, ones that require several types of calculations to arrive at the correct answer, like "given the IP address 10.10.0.0 devise an IP scheme that will allow for up to 500 hosts, but no more then 32 subnets". I though that out there, I didn't bother to look if it's even possible.

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Re: Subnetting

Fri Nov 12, 2010 2:48 am

Axis wrote:
Infinite wrote:One thing I tend to do a lot in my head is count the bits in binary. eg. 1 2 4 8 16 32 64 128. But actually, I do it backwards since this is all represented in little endian. 128 64 32 16 8 4 2 1.

I also find that I need to remember the addition as well. 128 192 224 240 248 252 254 255.

128+64=192
192+32=224
224+16=240
240+8=248
248+4=252
252+2=254
254+1=255

By memorizing those two sets of numbers I find I can calculate a given subnet fairly quickly.


Not bad, actually pretty helpful for doing it in my head without the chart...just take the sum, subtract it from 256 and you have your range of ips. So...

Normally when I do subnetting in my head without the "Magic Chart" I follow some crazy math logic like converting binary in my head

What are the subnet and broadcast IPs for host 10.65.231.31/22?

22 is 22 bits on 11111111.11111111.11111100.00000000
255.255.xxx.0
take the 6 bits on in the 3rd octet and add them up 128+64+32+16+8+4=252
256-252 = 4
So we know the ips are in multiples of 4
4x 60 is 240 which is too high, but close enough to know that if you take 2 away you're at 232 so make it 3 and your at 228.0 which is your subnet ip address and that makes 231.255 the broadcast


However doing it that way, I know the last bit on is the 6th so 248+4 = 252
256-252 = 4

Way less steps and less steps = less time, I'm down with that...thanks :)


I follow your logic. Usually I do very much the same.
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Re: Subnetting

Fri Nov 12, 2010 4:16 am

Hi Scott,

A quick tip for the exam to be fast with subnetting.

Sometimes you may be given a block of 4 or 8 (for example 10.1.1.202/30 [/30 is 6 bits – 128 + 64 + 32 + 16 + 8 + 4 = 252. 256 – 252 = 4] or 10.1.1.202/29 [/29 is 5 bits – 128 + 64 + 32 + 16 + 8 = 248. 256 – 248 = 8] ) and when you need to count up to 202 in 4's or 8's to find the first address of the subnet it can be time consuming (unless you’re good with maths, unlike me :)). So always remember that 4 always goes into 20, 40, 60, 80, 100 and so on. So using this technique straight away you know the subnet address will be 10.1.1.200, first address will be 10.1.1.201, broadcast 10.1.1.203 and the next subnet will start 10.1.1.204.

Likewise for 8, goes into 40, 80, 120, 160 etc.... This did help out in the exam world when you're on the clock.

Hope this helps.

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Re: Subnetting

Fri Nov 12, 2010 4:48 am

I break out the pen and paper quite often.

I also use a subnet app on my iphone and I also have Stretch's Subnet cheat sheet on my desk


It didn't take me too long to learn how to do the paper and pen route. When I was studying for my CCNA I just did hundreds of calculations. Got my wife to read out addresses and slashes and I would give back the hosts, subnet and broadcasts.

Same thing with supernetting. Supernetting is pretty easy actually

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Re: Subnetting

Fri Nov 12, 2010 4:54 am

mellowd wrote:I break out the pen and paper quite often.


Yeah me too but i stay away from using subnet calculators (unless i'm doing a huge design then i like to verify my work). I like to keep the formulas fresh in my head.

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Re: Subnetting

Fri Nov 12, 2010 8:07 am

grichardson661 wrote:Hi Scott,

A quick tip for the exam to be fast with subnetting.

Sometimes you may be given a block of 4 or 8 (for example 10.1.1.202/30 [/30 is 6 bits – 128 + 64 + 32 + 16 + 8 + 4 = 252. 256 – 252 = 4] or 10.1.1.202/29 [/29 is 5 bits – 128 + 64 + 32 + 16 + 8 = 248. 256 – 248 = 8] ) and when you need to count up to 202 in 4's or 8's to find the first address of the subnet it can be time consuming (unless you’re good with maths, unlike me :)). So always remember that 4 always goes into 20, 40, 60, 80, 100 and so on. So using this technique straight away you know the subnet address will be 10.1.1.200, first address will be 10.1.1.201, broadcast 10.1.1.203 and the next subnet will start 10.1.1.204.

Likewise for 8, goes into 40, 80, 120, 160 etc.... This did help out in the exam world when you're on the clock.

Hope this helps.


Good tips for finding SN/host faster on the real exams!
Make lots of sense to me.
Thanks Buddy!
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Re: Subnetting

Fri Nov 12, 2010 8:35 am

scottsee wrote:That's nuts. :shock:

I just take the IP 10.65.231.31/22 and think in terms of /8, /16. /24 where is it? /22 is two binary bits into the 3rd octet from the right, which makes the Subnet increment a multiple of 4, then just figure out the closest common multiple of 4 and 231, which 228. making 228.0 the network, 231.255 the broadcast and everything in the middle fair game.

What I have the most trouble with are story type question, ones that require several types of calculations to arrive at the correct answer, like "given the IP address 10.10.0.0 devise an IP scheme that will allow for up to 500 hosts, but no more then 32 subnets". I though that out there, I didn't bother to look if it's even possible.


Ahh, story subnets are the ones that gives you trouble...for practice for those I like to use the subnet game it's all story based subnetting

https://learningnetwork.cisco.com/docs/DOC-1802
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Re: Subnetting

Fri Nov 12, 2010 10:10 am

Axis wrote:Ahh, story subnets are the ones that gives you trouble...for practice for those I like to use the subnet game it's all story based subnetting

https://learningnetwork.cisco.com/docs/DOC-1802


It's funny you bring that up, I've been playing that game for about a week. I like the Cisco Arcade. To be honest, I'm actually pretty decent at algebra and pre-algebra math which comes from helping my 11 year old daughter do her math homework every night. The stuff they expect kids to know is a lot different then when we were kids!! I don't recall doing half the stuff she does until I was in the last year of middle school..

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Re: Subnetting

Fri Nov 12, 2010 10:26 am

scottsee wrote:It's funny you bring that up, I've been playing that game for about a week. I like the Cisco Arcade. To be honest, I'm actually pretty decent at algebra and pre-algebra math which comes from helping my 11 year old daughter do her math homework every night. The stuff they expect kids to know is a lot different then when we were kids!! I don't recall doing half the stuff she does until I was in the last year of middle school..


Scary huh? Kids are coming out of school with CCNA certifications.
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Re: Subnetting

Fri Nov 12, 2010 12:33 pm

IP Addressing and Subnet Workbook Student Version 1.5

I preferred something to write out. Save trees and all that but it helped me.
- Pete

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Re: Subnetting

Fri Nov 12, 2010 12:41 pm

I think subnetting (especially subnetting fast) is an overrated skill. It's just basic mathematics, and outside the exam environment, you don't need to do it fast, unless you're trying to impress someone. Just give it a minute, a pen and a paper; no need to rush.

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Re: Subnetting

Fri Nov 12, 2010 1:22 pm

Project2501 wrote:IP Addressing and Subnet Workbook Student Version 1.5

I preferred something to write out. Save trees and all that but it helped me.


Wow, great link, that's a whole lot of subnetting! :cheers:

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