Subnetting Practice

fieldmonkey

Ok... I am going to time myself at completing the Problem Set 5: Analyzing an Address in an Existing Subnet and documenting all items. I have learned thus far after I get the Network # and the Broadcast #, I can basically draw any other needed information, at a glance, for multiple choice scenario. However I need to master this, in for SIMS etc.

So I thought I would create this post for fun and track my scores (total / average) for the 25 IP and subnets mask in the problem. According to what I have read, I should be able to complete this information in no more than 30 seconds for each problem or just over 9 minutes in total.

Right now I am well above that. Feel free to suggest other methods, as I am using the "interesting octet / magic number" method.

All I have done in advance is created some practice sheets that will keep me from having to rewrite the targeted information being obtained from each IP address and subnet mask.

Network Bits:
Host Bits:
Subnet Bits:
# subnets:
# host / subnet:

Broadcast:

First IP:
Last IP:

When I am done I will post my time... Ready, Set.... GO!

Xenz

I'm not sure about the method, it could be the one I use, but subnetting for me is pretty easy until you get into the high subnet masks /30 takes a couple seconds to find the network.

All that information you have listed is fine, but in reality you can skip most of it. Finding the network the host belongs to and using the mask you can determine all that in your head fairly quick. So I would suggest slowly getting away from writing everything down. Otherwise you'll continue to do it in your head probably and slow yourself down.

My only advice to you is find a way subnetting works for you and DO NOT CHANGE. So many people get confused when you teach them multiple ways to do it. Find whatever method works and stick with it, really, stick with it.

fieldmonkey

Xenz wrote: »

All that information you have listed is fine, but in reality you can skip most of it. Finding the network the host belongs to and using the mask you can determine all that in your head fairly quick. So I would suggest slowly getting away from writing everything down. Otherwise you'll continue to do it in your head probably and slow yourself down.

Ya know I thought about this, but felt I needed to suffer by going about the manual process, so that I might gain a deeper understanding by working thru the mechanics of breaking it down quickly and compiling all the information in the excercise.

I definitely agree that getting the subnet address can be done quickly, and from that information you can draw your answer very fast. Recording the information eats up alot of time.

What I am confused about is in the Wendell book it mentions being able to gain this information within 15-30 seconds, I guess I am overthinking it?

However I wanted to put myself to the test of obtaining and documenting all 25 problems and I am embarassed to say it took me 1:11 minutes (2.8 min/?). Maybe I'm my own worst critic. I did document everything in that time and only missed one subnet out of the 25, and a few glitches on identifying a couple of ranges or broadcast numbers (off by 1).

Tuff excercise --- onward to Problem Set 6 now

miller811

here is a good site to sharpen/test your skills

IP Subnet Practice

kevin31

rpfutrell wrote: »

Ya know I thought about this, but felt I needed to suffer by going about the manual process, so that I might gain a deeper understanding by working thru the mechanics of breaking it down quickly and compiling all the information in the excercise.

I definitely agree that getting the subnet address can be done quickly, and from that information you can draw your answer very fast. Recording the information eats up alot of time.

What I am confused about is in the Wendell book it mentions being able to gain this information within 15-30 seconds, I guess I am overthinking it?

However I wanted to put myself to the test of obtaining and documenting all 25 problems and I am embarassed to say it took me 1:11 minutes (2.8 min/?). Maybe I'm my own worst critic. I did document everything in that time and only missed one subnet out of the 25, and a few glitches on identifying a couple of ranges or broadcast numbers (off by 1).

Tuff excercise --- onward to Problem Set 6 now

Hi

Can you post how you do your subnetting please?

thanks

kev

fieldmonkey

kevin31 wrote: »

Hi

Can you post how you do your subnetting please?

thanks

kev

Well, it is kind of hard to show on here, but if you have the Cisco Press Guide it's in the book already under Chapter 12 and the appendixes.

I just googled "interesting octet" to see what came up and this might help explain the method.
____________________________________________________________

Interesting OCTET = octet in subnet mask with out a 0 or a 255

Increment = 256-subnet mask in interesting octet
Current Subnet = IP address in interesting octet /Increment

Then: Increment * (answer round to nearest whole number)
Next Subnet = Current Subnet + Increment
Broadcast Address = Next Subnet - 1
1st IP Address = Current Subnet + 1
Last IP Address = Broadcast Address - 1

Xenz

30 seconds should be the max you should shoot for. Once you get enough practice you can do them in 5-15 seconds. I think Odom was commenting on the time constraints for the CCNA test. taking more than 30 seconds to subnet can eat up time you need to finish the test in time.

I'm teaching my cousin and he takes a long time still after a month. Just practice 5-10 a day for a couple months and you'll have it down.

*note* I work out my problems using the interesting octet method I guess. If you want to speed things up, instead of writing out everything just write out the numbers for the octets that change so it looks like:

192.168.1.0 /26
.1 - .62
192.168.1.64 /26
.65 - .126
192.168.1.128 /26
.129 - .190
192.168.1.192 /26
.193 - .254

This saves you some time because you're not writing out the whole IP for the first/last. If you wanted to save even more typing just write out the network numbers too. Just don't get confused.

Solaris_UNIX

miller811 wrote: »

here is a good site to sharpen/test your skills

IP Subnet Practice

Thanks for that link miller811. It's a really useful tool for finding weaknesses in one's subnetting capabilities.

I gave you +1 karma / reputation to show my appreciation for it.

By working on some randomly generated problems at that link, I've discovered that I"m really fast at subnetting anything from a /24 on down but I'm horribly bad at anything that's a /23 on up...

I've been working on Linux and Unix sysadmin jobs for a while now and it's gotten to the point where I just see "/25" and automatically know it's a 255.255.255.128 netmask and that it's two subnetworks without even thinking about it or having to do any binary math and that "/26" is 255.255.255.192 with four subnetworks etc. but if I get into anything above a Class C or /24 then I start to slow down significantly as I've never deployed a server in real life that was on a /18 subnet (you would think all the ARP broadcast traffic in such a big subnet would slow it down a lot eh ). So yeah, I suck pretty bad at subnetting those really big Class A and Class B ranges (from /8 to /23) which is bad for me because those are the ones that you will probably have to deal with for really large scale things like BGP routing between multiple ASN's on the internet

For example, say I got this question- how many host IP addresses in a /19?

Heck if I know.... I would have to get out a pen and paper and work on that one for a bit. However, it's VERY important to know the answer to that question in real life though as ARIN gave out a lot of /19 subnets back in the day and you might end up working for an ISP company that has one.

Xenz

/19 =

2^13 8192 - 2
4096
2048
2^10 = 1024 <-- I start from here because I know /22 is 1024 - 2 for hosts

5 network bits = network blocks of 8
148.0.0.0 - 148.7.255.255

Just try to find a decent start and remember how to count. In reality, if you're given a mask like that you're not going to make any hasty decisions on subnetting anyways so it's not a big deal if you can't do it within 15 seconds.

fieldmonkey

Solaris_UNIX wrote: »

I've been working on Linux and Unix sysadmin jobs for a while now and it's gotten to the point where I just see "/25" and automatically know it's a 255.255.255.128 netmask and that it's two subnetworks without even thinking about it or having to do any binary math and that "/26" is 255.255.255.192 with four subnetworks etc.

Oh man, I can't wait to get to this point where I SEE it (resident subnet, # subnets, ip ranges and broadcast) just by looking at the prefix mask!

I can do it all on paper, gather the data then give you an answer, but to just LOOK at the prefix mask and SEE it! ... WHOA! that's just too cool!

miller811

rpfutrell wrote: »

Oh man, I can't wait to get to this point where I SEE it (resident subnet, # subnets, ip ranges and broadcast) just by looking at the prefix mask!

I can do it all on paper, gather the data then give you an answer, but to just LOOK at the prefix mask and SEE it! ... WHOA! that's just too cool!

Have you checked out this technique?

http://www.techexams.net/forums/ccna-ccent/38772-subnetting-made-easy.html

That is all I required to solve the mystery of subnetting.

Hope it helps you.

Kaminsky

rpfutrell wrote: »

Oh man, I can't wait to get to this point where I SEE it (resident subnet, # subnets, ip ranges and broadcast) just by looking at the prefix mask!

By then you will be sick to back teeth with subnetting.

Keep doing it manually and hitting subnettingquestions.com to find your holes and it will come.

I had a really boring temp job where I had done most of the work but they wanted to keep me around for another 6 months just in case. During a work day I was doing an hour of questions in my head, 3 times a day for weeks and weeks. Speed got to be phenominal. You just get used to it but you have to learn it by doing it manually.

jason_lunde

I go to subnettingquestions.com - Free Subnetting Questions and Answers Randomly Generated Online to sharpen my skills about once per week. Especially useful right before an exam! Good luck.

billscott92787

Try these two subnetting practice tests:


Subnetting Quiz #1


Subnetting Quiz #2 (CIDR)


I used #2 to get use to doing things in my head. At first, I was doing everything on paper. Then, I used the windows calculator to add up the digits in binary. Then eventually, I was able to do it in my head without using paper or a calculator. I honestly practiced everyday for three months. Once you get use to the specific masks, you'll be able to recognize the patterns of how each address ends like the network and broadcast addresses, then the range. I'm not sure how your text recommends it. But how I learned was take the mask:

/25 which would be 11111111.11111111.11111111.10000000 The last octect is the one that we are interested in so 1000000 or 128:


Determined by:
128 64 32 16 8 4 2 1

1 0 0 0 0 0 0 0 as you can see there is only one "on" bit which is in what position? 128.

calculate the subnet block: 256 - 128 = 128, from this you can calculate each subnet. Lets use the network 192.168.2.0

We have a total of 7 host addresses left. This means that we have 2^7-2 = 128-2 = 126 host addresses per subnet, plus one is reserved for the network and one is reserved for the broadcast. (128 total).

192.168.2.0 /25 what do you think the next will be?

192.168.2.128 /25

192.168.3.0 /25

192.168.3.128 /25

192.168.4.0 /25

192.168.4.128 /25 and so on

fieldmonkey

guys--

quick update!

I have seen the light and it's all coming together .... thanks for the great links to continue testing my quickness.

I think tonight after my awakening, I can definitely git-r-done in under 30 seconds very soon. I am currently flying through them a warp speed, as compared to the way I originally posted....

Could I be on my way to final preparations and booking my CCENT???

Let's hope so... promised myself I would have an exam date booked by months end.

miller811

rpfutrell wrote: »

guys--


Could I be on my way to final preparations and booking my CCENT???

Let's hope so... promised myself I would have an exam date booked by months end.

Nothing motivates me more than setting a date and scheduling the exam.
Then it gets all of my attention.

fieldmonkey

miller811 wrote: »

Nothing motivates me more than setting a date and scheduling the exam.
Then it gets all of my attention.

booked"!


MOST RECENT BEST/WORST TIMES


BEST: :39

172.31.140.14/25
172.31.140.0/1 first
172.31.140.255/254 last

WORST: 1:59

192.168.100.100/28
192.168.100.96/97 first
192.168.100.111/110 last

Solaris_UNIX

rpfutrell wrote: »

booked"!


MOST RECENT BEST/WORST TIMES


BEST: :39

172.31.140.14/25
172.31.140.0/1 first
172.31.140.255/254 last

WORST: 1:59

192.168.100.100/28
192.168.100.96/97 first
192.168.100.111/110 last

Good job!

How I got used to visually "seeing" how to subnet /24's instead of just doing binary math (how I used to do it) happened when I was tasked to manually map out several dozen /24 subnets worth of server IP addresses with hostnames in Open Office / Microsoft Excel spreadsheets (i.e. each /24 was it's own spread-sheet document with a network ID and router IP address up at the top, a broadcast at the bottom, and the names of servers with their IP addresses in the middle).

We then did a Python script that read all of the IP addresses and hostnames from the spreadsheets and loaded it into a database that told us in which datacenter and in which cabinets certain servers were located in based on things like IP address and Mac address. It was a big project, but it got me used to doing class C subnets.

The easy way to remember how to divide up a class C in your head is like this:

biggest class C is:

/24 = 255.255.255.0 (there's no way should forget that one)

smallest class C is:

/30 = 255.255.255.252 (this one's used for point to point WAN links, so you better memorize it).

then kind of understand the pattern to move up from a /30:

252 - 4 = 248 /29
248 - 8 = 240 /28
240 - 16 = 224 /27

notice that the number I'm subtracting increases by powers of two ( 4, 8, 16, 32, etc.)

or down from a /24:

128 + 64 = 192 /25
192 + 32 = 224 /26
224 + 16 = 240 /27

Hope that information helps and doesn't confuse anybody.