On the brink of mastering quick subnetting

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AMD_Gamer

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I have been struggling for a long time on how to master subnetting where I can do it quickly for the exam and without a calculator. Over the past few days I have read and almost mastered this method: http://www.techexams.net/forums/ccna-ccent/38772-subnetting-made-easy.html

This has been a huge confidence boost and the only obstacle I see on my path to my CCNA.

I have been doing the questions at http://www.subnettingquestions.com/ and getting most of them correct using a pencil and scrap paper very easily.

If I have been doing that good is that all I should need? does subnettingquestions.com cover what you would see on the exam and need to know how to do?

Does anyone else use this method or have any tips?
 
My advice is just use whatever method works for you.

Personally I've used the "make yourself a chart before you start the exam questions and the timer begins counting down" method for the last ten years since it has always worked well for me. As time has passed the chart has been enlarged, wild card masks added, etc as I moved through different exams.

Important things to remember are how to calculate max hosts per subnet, how to summarize a few prefixes into a single prefix, that the first and last address of each subnet are reserved and cannot be used by hosts (on Cisco exams at least), given a network and mask length whether an IP address falls into or resides outside the network. etc... If IOS ACL questions are involved make sure you pay attention to mask bits properly with wildcard masks since Cisco likes to be sneaky on these occasionally.

That's about all I can think of at the moment. Just don't waste time fully calculating the right answer for all the subnetting questions. Most of the time you can eliminate 2-3 answers right away because of a glaring error. You then only have to decide between the 2 or 3 left usually which can be done with minimal calculations.
 
Check out the practice subnetting page on my forum. It presents a randomly generated subnetting problem and provides the answer and an explanation of how to solve the problem.

I'd love to get some feedback on it so let me know what you think.

Steve

ps Hi Vito!
 
subnetting was easy and wasn't a factor on the CCNA exam. they usually just pick a few ranges and thats pretty much all your asked about like /28 and /29 or a /30
 
Vito_Corleone said:
Huh?
Click to expand...
"In common usage, the "host all zeros" address is reserved for referring to the entire network, while the "host all ones" address is used as a broadcast address in the given subnet; this reduces the number of addresses available for hosts by 2. This explains the reference to /31 networks as "Rarely Used," as the only possible addresses on a /31 network are "host all ones" and "host all zeros." RFC 3021 creates an exception to the "host all ones" and "host all zeros" broadcast usage to make /31 networks usable for point-to-point links. In practice, however, point-to-point links are still typically implemented using /30 networks, or occasionally by /32 and point-to-point explicit host routes.[citation needed] There is generally no technical advantage to /31 versus /32, although one or the other may be more convenient based on other issues. A /30 is always wasteful and has as its sole advantage that it behaves "as expected" for any other subnetwork."
 
mattjw916 said:
"In common usage, the "host all zeros" address is reserved for referring to the entire network, while the "host all ones" address is used as a broadcast address in the given subnet; this reduces the number of addresses available for hosts by 2. This explains the reference to /31 networks as "Rarely Used," as the only possible addresses on a /31 network are "host all ones" and "host all zeros." RFC 3021 creates an exception to the "host all ones" and "host all zeros" broadcast usage to make /31 networks usable for point-to-point links. In practice, however, point-to-point links are still typically implemented using /30 networks, or occasionally by /32 and point-to-point explicit host routes.[citation needed] There is generally no technical advantage to /31 versus /32, although one or the other may be more convenient based on other issues. A /30 is always wasteful and has as its sole advantage that it behaves "as expected" for any other subnetwork."
Click to expand...

pretty much subnetting 101... the binary bit is easy imo... split them into groups of 8 and it's simple to understand...

i remember having "that epiphany" when taking my first few CCNA classes...

i also had heavy programming in high school so converting binary to decimal (and vice versa) was something i could already do easily....
 
AMD_Gamer said:
what about VLSM?
Click to expand...

all the same stuff, you're just subnetting a subnet. If I had more time I would photoshop an xzibit picture with the caption "VLSM - yo dawg, I heard you like subnetting, so I put a subnet in your subnet"

EDIT: apparently not the first to use that meme:
Yo-dawg-I-herd-you-liek-subnetting-So-i-put-a-subnet-in-your-subnet-so-you-can-subnet-while-youre-su.jpg
 
VLSM, CIDR, etc all go hand-in-hand now. There really isn't anything "classful" left in modern networks. It's all prefixes and mask length. Most people just assume /24 when doing private routing because it's convenient and easy to remember, e.g. the last octet is the host address, the rest is the network.
 
se4b4ss said:
Check out the practice subnetting page on my forum. It presents a randomly generated subnetting problem and provides the answer and an explanation of how to solve the problem.

I'd love to get some feedback on it so let me know what you think.

Steve

ps Hi Vito!
Click to expand...

great stuff, i did a bunch of them but there is only one kind of question asked and not much variation?
 
AMD_Gamer said:
great stuff, i did a bunch of them but there is only one kind of question asked and not much variation?
Click to expand...

from what i remember, there weren't really any straight subnetting questions. It was more scenarios where you had to work with subnets and match up different networks with different pieces of hardware...more like verifying information than actually doing it.
 
Vito_Corleone said:
Ah, okay. Your post confused me.

@cooper, I don't think /31s are subnetting 101. Many people don't even know you can use them.
Click to expand...

not in my course.. iirc we used them a few times on serial links when setting up routers in the lab.... i guess my prof was a little more thorough than most....
 
AMD_Gamer said:
great stuff, i did a bunch of them but there is only one kind of question asked and not much variation?
Click to expand...

Yeah, the variation is in the subnet and mask, not the type of question. It was hard enough to code without adding multiple question types. :)

Hopefully it will help folks with the math and in seeing the patterns.
 
what do you guys to to quickly increment up to the subnet when the block size is really small like 4 or 8?
 
Cisco has a subnetting game that i hear helps out a lot, close friend used it and he liked it. Might want to give it a look
 
AMD_Gamer said:
what do you guys to to quickly increment up to the subnet when the block size is really small like 4 or 8?
Click to expand...

i have the same question...... starting from 0 to find the range of a subnet somewhere near the 100s is time consuming with small block sizes.

any shortcuts?
 
AMD_Gamer said:
what do you guys to to quickly increment up to the subnet when the block size is really small like 4 or 8?
Click to expand...

Use easy multiples of the block to get into the range you need to be faster, ie, count up by 16, 32, 64, 128, etc. Say for example you have 192.168.1.146/29, and need to know what network it falls into. /29 = increments of 8, so we can just use a multiple, 128, to get up into that range quickly. From there you can count by 8, or add another easy multiple to it, like 16, which takes us to 144, the start of our subnet.
 
nexusalt said:
Use easy multiples of the block to get into the range you need to be faster, ie, count up by 16, 32, 64, 128, etc. Say for example you have 192.168.1.146/29, and need to know what network it falls into. /29 = increments of 8, so we can just use a multiple, 128, to get up into that range quickly. From there you can count by 8, or add another easy multiple to it, like 16, which takes us to 144, the start of our subnet.
Click to expand...

ahhh ok. makes sense. thx!
 
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