Help With CCNA Question - VLSM/Subnets

[Anthonyuk]

Hi,

I have been reading every text book I have and googling everything I can think of for the last two days but I can not for the life of me find any useful info on how to tackle this, likely simple to those in the know, question I have for my CCNA course.

I don't want someone to give me the answer as there is no point it this, but I would like some help on material giving relevant examples on figuring it out.

Ok the question is in three parts:.

Given the class A network 10.0.0.0 create a subnet mask for the """ subnets.

(I have put """ as I don't want help in answering this specific number of subnets, but its a large number).

Then it asks identify the """th (also a very large number).

Then VLSM the """th (also a large number)

From reading Lammles book all the examples I can find start with the Subnet mask and work from there forward.

I can not find anything in the book on starting from the network to obtain a subnet mask for only a certain subnet amount.

I have read the default subnet for 10.0.0.0 is 255.0.0.0 and worked this out into 11111111.0000000.00000000.00000000 into binary.

But where to go now?

Obviously it will be easy to people that know this but like mathematics, for my mindset at least, it is impossible to see how something is done without seeing similar examples before tackling your own.

Thanks for any help anyone is able to provide.

[hbroomhall]

Hi and welcome to CF!

The standard answer often given here is to look at this site for a tutorial on subnetting.

Harry.

[JonGlory]

I dont know if i picked up the question right but to get the number of bits you need to create x amount of subnets, just think of it like this.

If i borrow 1 bit i get 2 subnets
if i borrow 2 bits i get 4 subnets
if i borrow 3 bits i get 8 subnets
if i borrow 4 bits i get 16 subnets

see the trend?

add 1 to the bits and times the subnets by 2

[CheeseOnToast]

well, the guys above are heading in the right direction

But with VLSM you are actually taking the IP you are given and breaking it down in hierachal fashion to use your address resources effectively without waste

and yes you are assigning a subnet-mask which is appropriate to the situation

To be honest, I have only learned it myself the past week or so, and I don't think it is the type of thing that can be taught on the forum, I would google it and google some VLSM practise papers and basically just keep going!

here is a small eg -

Take Class C address 192.168.10.0

If you subnetted this using a /26 mask, you would be left with 62 useable hosts per subnet (6^2-2).

So lets say you use subnet 0 for your first subnet, and you use this on a P2P serial link between two routers, you would use 2 addresses, wasting the other 60 addresses!

This is the point of VLSM.

You could take subnet 0, use the 62 hosts for say your PC's in one building. Take the following subnet, which in this case, would be 192.168.10.64, and then further subnet it, e.g. using a /27 mask if your other building only needed 30 useable hosts... then take the further subnet e.g. 192.168.10.128 and subnet it with a /30 subnet mask, yielding 2 useable hosts for your P2P link...

Do you see the kind of point I'm trying to make>?

Maybe my eg is crap - but you need to read up and do some practise ones to be honest and come back with any specific q's

Sorry to anyone if I've made any mistakes in that above!! lol

[sunn]

I can’t tell if this is a subnetting question or a summarization question.

Subnetting :
These are aspects pertaining to subnetting:
(i) Number of Subnets? // (2ⁿ) -where n is number of 1’s (ip subnet zero is used)
(ii) The specific subnets? // (256 – subnet mask) = base number (starting point)
(iii) Broadcast for each subnet? // The address preceding next subnet
(iv) Number of hosts per subnet? // (2t - 2) where t is number of 0’s
(v) The Hosts in the subnet? // Hosts are the addresses between the subnets (dropping the network and broadcast address or all 0’s and all 1’s respectively)

Example:
206.235.255.78 255.255.255.240 (or '/28')

Number of subnets = (2^4) = 16
What subnets = (256 – 240) = 16. Subnets are 0 | 16 | 32 | 48 | 64 | …
Number of hosts / subnet = ((2^4)-2) = 14

Network: 206.235.255.0 Network: 206.235.255.16
Host Range: 206.235.255.(1-14) Host Range: 206.235.255.(17-31)
Broadcast: 206.235.255.15 Broadcast: 206.235.255.32

[Yido]

In the question it should state how many networks are needed or how many hosts are required.
This is usually done with a prefix or slash notation (/) to define the subnet mask.
If they want you to use the default subnet mask then this is unsubnetted and in effect is just 1 large network.

Dont think youve put the whole question down unless im getting the wrong end of the stick.

[sunn]

Here's the summarization example:
Disclaimer:
i) This is a crude example, but it gets the point accross. Crude becuase the summarization is too broad.

Take the following network addresses:
192.168.98.0-----192.168.99.0----192.168.100.0----192.168.101.0
192.168.102.0----192.168.103.0---192.168.104.0----192.168.105.0

1) Convert the addresses to binary format and align them in a list.

--Address--------1st Octet--2nd Octet--3rd Octet--4th Octet
192.168.98.0------11000000 10101000 01100010 00000000
192.168.99.0------11000000 10101000 01100011 00000000
192.168.100.0-----11000000 10101000 01100100 00000000
192.168.101.0-----11000000 10101000 01100101 00000000
192.168.102.0-----11000000 10101000 01100110 00000000
192.168.103.0-----11000000 10101000 01100111 00000000
192.168.104.0-----11000000 10101000 01101000 00000000
192.168.105.0-----11000000 10101000 01101001 00000000

2) Find where the common pattern of digits ends. (i.e. 11000000 10101000 0110xxx xxxxxxxx) x's are uncommon bits
3) Count the number of common bits. The summary route number is the 1st IP address in the block, followed by the mask (or CIDR notation).

Answer:
Summarized route is 192.168.96.0/20

192.168.96.0 11000000 10101000 01100000 00000000

[CheeseOnToast]

Funny, I've never been shown before that you can do 256 - Subnet Mask to get the starting point of the subnet ranges

Bloody useful one to know!

I always end up quickly jotting the binary down lol

[sunn]

Originally Posted by CheeseOnToast

Funny, I've never been shown before that you can do 256 - Subnet Mask to get the starting point of the subnet ranges

Bloody useful one to know!

I always end up quickly jotting the binary down lol

Doing it in binary will obviously work, but you'll probably run out of time during an exam. Use the 256 (magic number) method and you'll save precious time.

[CheeseOnToast]

Originally Posted by sunn

Doing it in binary will obviously work, but you'll probably run out of time during an exam. Use the 256 (magic number) method and you'll save precious time.

I repped you kind sir

[r.h.lee]

Originally Posted by Anthonyuk

Hi,

I have been reading every text book I have and googling everything I can think of for the last two days but I can not for the life of me find any useful info on how to tackle this, likely simple to those in the know, question I have for my CCNA course.

I don't want someone to give me the answer as there is no point it this, but I would like some help on material giving relevant examples on figuring it out.

Ok the question is in three parts:.

Given the class A network 10.0.0.0 create a subnet mask for the """ subnets.

(I have put """ as I don't want help in answering this specific number of subnets, but its a large number).

Then it asks identify the """th (also a very large number).

Then VLSM the """th (also a large number)

From reading Lammles book all the examples I can find start with the Subnet mask and work from there forward.

I can not find anything in the book on starting from the network to obtain a subnet mask for only a certain subnet amount.

I have read the default subnet for 10.0.0.0 is 255.0.0.0 and worked this out into 11111111.0000000.00000000.00000000 into binary.

But where to go now?

Obviously it will be easy to people that know this but like mathematics, for my mindset at least, it is impossible to see how something is done without seeing similar examples before tackling your own.

Thanks for any help anyone is able to provide.

Anthonyuk,

Check out the following link.

Link:

1. IP Addressing and Subnetting for New Users - Cisco Systems - http://www.cisco.com/en/US/tech/tk36...800a67f5.shtml

[purplejade]

Normally in CCNA exams you will have atleast 2 to 3 questions on VLSM & Summarisation, these will be of pick and drop sort questions where you have a network diagram giving you the no of hosts in each network, and you have to allocate the address from the given choices. the address will all have cidr notations, and as long as you know your blocksizes and work out quickly the no of subnets and hosts for each given ip address it should be fairly simple and easy..you can find practice examples in cd accompanying the ccna press books...