How would you subnet in your head?

StonedHitman

So, i really need to learn how to subnet in my head, what is your guys' strategy for this? do you just work it out in your head as if you were writing it on scratch paper? This is how i would subnet if i were writing it down......

Subnet 200.10.10.0 so that i have 16 usable hosts.

200.10.10.0 - Address
255.255.255.0 - Default Mask
11111111.11111111.11111111.00000000 - Default Mask converted to binary
11111111.11111111.11111111.000/00000 - borrowed 3 bits(converted to network bits) 8 total subnets/32 total hosts(30 usable)
255.255.255.224 - Custom Subnet Mask

Im guessing i would have to memorize this step by step in my head, or is there an easier method?

bobfromfpl

The more you do it, the more it comes natural. Everyone has a method that works for them, so if someone tries to explain it to you one way and it just isn't clicking then search for another technique! Its difficult at first but it'll be fluent in due time.

Jon_Cisco

I agree there are a bunch of different ways to do it and whichever works best for you go for it.

About a month ago I sat down for an hour straight and just did subnetting questions using subnetting.net - Subnet Questions and Answers

As you work through the problems they will start to get easier. When your almost done pay attention to what your thought process is. Take few notes about it. Come back to it a day later and see if you can do it easier.

The best advice I can say is memorize the main numbers you will be working with.
2 4 8 16 32 64 128 256 and 128 192 224 240 248 252 254 255

Gngogh

hi,

look at this post

http://www.techexams.net/forums/ccna-ccent/97487-subnetting-jeremy-cioara-method.html

search for youtube videos, and find the MAGIC NUMBER.

TechGuru80

It is actually 255.255.255.240 not .224. Remember 2^x = amount of networks, and 2^(8-x) = hosts per network. So if you need 16 hosts per network, 2^4 = 16 networks, 2^(8-4) = 2^4 = 16 hosts per network.

128+64+32+16 = 240. 4 bits for the network, 4 bits for the host.

Bellman80

TechGuru80 wrote: »

It is actually 255.255.255.240 not .224. Remember 2^x = amount of networks, and 2^(8-x) = hosts per network. So if you need 16 hosts per network, 2^4 = 16 networks, 2^(8-4) = 2^4 = 16 hosts per network.

128+64+32+16 = 240. 4 bits for the network, 4 bits for the host.

Eh, 16 usable hosts means he can't use 2**4, it has to be 5 bits

To the OP:
Here is my thought process fwiw:
16 hosts - has to be 32 (because you have to subtract 2), so hosts will take 5 bits (this is where knowing powers of 2 comes handy)
the networks are left with 3 bits, so only 8 networks
the first 3 bits are 128, 64, 32. Sum them up for 224 (or 24+3 if it's in the / format)

TechGuru80

Ah sorry it was late in the day lol. I meant range of 16 (14 hosts)...correct 3 bits 8 networks, 32 range (up to 30 hosts)...224.

Clevernamehere

I would just say 2^4=16. 32-4=/28 or 255.255.255.240. That is if you need 14 hosts of course.

goldenlight

using the Magic Number Trick

learn to count in multiples of the magic number 128 64 32 16 8 4 2 1

subnet mask 128,192,224,240,248,252,254,255

ex 0, 128, 255
0,64,128,192,255

0,32,64,96,128,160,192, 224,255


etc



magic number trick got me through subnetting.. While I was reading the exam instructions. I was building my subnet chart..

johnwest43

IMHO I would stay away from the binary methods. Simple subtraction and addition most times will do the trick.
Example: What network does the address 10.1.1.93 255.255.255.224 belong to?

Step 1 locate the interesting octet from the mask first. (224)
Step 2 use the answer from step 1 to locate the interesting octet from the ip address (93)
Step 3 subtract 224 from 256 (256-224 = 32)
Step 4 add the answer from step 3 to itself until it is greater than the interesting octet of the ip address (32+32+32=96 , 96>93)
Step 5 subtract the answer from step 3 (32) from the answer of step 4 (96) 96-32= 64
Answer: the network address is 10.1.1.64

Once you practice for awhile you will find short cuts that will lead you to the answer in no time flat. I made sure I could subnet in my head before attempting the CCENT. It was well worth the effort.

Also if you can memorize the "slash notation to subnet mask" table.
/24 = 255.255.255.0
/25 = 255.255.255.128
/26 = 255.255.255.192
/27 = 255.255.255.224
/28 = 255.255.255.240
/29 = 255.255.255.248
/30 = 255.255.255.252
/31 = 255.255.255.254
/32 = 255.255.255.255

Adam B

StonedHitman wrote: »

So, i really need to learn how to subnet in my head, what is your guys' strategy for this? do you just work it out in your head as if you were writing it on scratch paper? This is how i would subnet if i were writing it down......

Subnet 200.10.10.0 so that i have 16 usable hosts.

200.10.10.0 - Address
255.255.255.0 - Default Mask
11111111.11111111.11111111.00000000 - Default Mask converted to binary
11111111.11111111.11111111.000/00000 - borrowed 3 bits(converted to network bits) 8 total subnets/32 total hosts(30 usable)
255.255.255.224 - Custom Subnet Mask

Im guessing i would have to memorize this step by step in my head, or is there an easier method?

kk, this is the way I go about it, and it makes total sense, at least in my head haha. 16 usable hosts, 200.10.10.0 address, that's a class c since they didn't specify it otherwise. Regardless, 16 usable, start going through your block sizes in your head. these are key to memorize. 255.255.255.240 does give you a block size of 16, but 16-2=14, so that's not 16 usable. 255.255.255.224 is block size of 32, allows for 30 usable, 16 falls in that category, don't want to use for example a 255.255.255.192, although possible, because you want to save as much address space as possible to be efficient(no need for 62 usable when you can have 30 usable for the 16 hosts you need basically). so block size of 32, bam find your networks ect ect. Overall, as I mentioned before, just memorize those block sizes and practice till its down pat in your head. Good Luck hope this made sense

kmusk01

If you have an android devices download the app: Binary Blitz Its a great little game that will help you with subnetting. I try to 5-10 minutes a day on my break. It truly does help. Also as mentioned before try to stay away from binary. Just remember block sizes, and a lot of it can be done in your head. The more you practice the more it will just "click".

Ken~