Subnetting...

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Hey guys

I have reached the dreaded topic of subnetting...

Basically im using the Mike Meyers book, network +, and for those with the book, i am on page 316. And I am totally lost. I understood up to page 315, but after this it jumps up to very complex stuff which doesnt make sense to me!

I know from the N+ syllabus we need to know the purpose of subnetting..

What i know is...

Subnet mask defines which part of the IP is the network ID and which is the host ID, to determine if the IP is local or remote

Class A = 1-126 - 255.0.0.0
Class B = 128 - 191 - 255.255.0.0
Class C = 192 - 223 - 255.255.255.0

I also know how to convert the numbers to binary and back again, and the naming convention like this: 192.168.100.1/24

The bit im on keeps alking about taking binary numbers and moving the mask from 0 to 8 places from the dot

As far as i thought, the subnet mask defines which bit is the network ID by having 255, and which is host, by having that as 0....

Granted I havent finished the chapter yet, but was hoping someone could enlighten me on if what ive put above is correct, and if this is enough knowledge for the N+?

Many thanks

 

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Firstly, the subnet mask defines which part of the IP address is the network ID by identifying it with a series of *1s* (in binary)

So,

11111111 . 11111111 . 11111111. 00000000
255 .............255.......... 255..........0

Looking at the last octet (zeros), the above allows for up to 254 hosts; 2^8 -2 (the minus two is the broadcast address *all 1s* and the network ID *all zeros*).

You can see that the first three octets are the network ID and the last (all zeros) are the available hosts.

*Subnetting* is moving on from this basic masking scheme where the whole octet is either 1s or 0s.

You would do this if you need more than 254 hosts, as below..

11111111 . 11111111 . 11111110. 00000000
255...............255.............254........0

Now you have 9 bits to play with for your hosts, which gives you..

2^9 -2 = 510 (up to 510 possible hosts)

And so on, each time you steal a *1* from the network ID you reduce the possible amount of subnets but you increase the possible max amount of hosts on each subnet.

I would recommend you go to http://www.learntosubnet.com and go through some of their exercises.

Pete

 

I am having a bit of trouble with subnets as well. I am using the Meyers Passport and the Sybex books to study for the N+, and they seem to describe the procedure of subnetting in opposite ways -- The Meyers book does it as mentioned and further explained in this thread, borrowing bits from the network portion of the subnet mask to break the large network into smaller networks. I also understand the way the Sybex book describes the process, which is to ADD bits to the host portion of the subnet mask i.e. 255.255.255.128 (11111111.11111111.11111111.10000000) which would break the network into four subnets with 62 IP addresses per subnet not counting the network and broadcast addresses. Possibly this is the only way of doing it on a class C address and keep it strictly class C?

Anyway, my main question is if subnetting features heavily on the N+ exam, which method (if any) of the two above is the "CompTIA way"?

 

Thanks for your help guys!
Printed off your posts and read through them very slowley. I think the problem i had was i was thinking of the mask in decimal only, when i looked at it in binary, as per your guides, it made a bit more sense

Unfortunatly www.learntosubnet.com doesnt seem to be working

Going to try the rest of the chapter now

Trogdor - I think the N+ only needs to know the purpose of subnetting, so I guess what we are covering is slightly more advanced, but its good to learn a bit more then you need to know sometimes, helps you understand what you do need to know

 

Although the site is down at the mo, i have only been able to view it with IE in the past.

Si

 

Thanks for that, guess I should have consulted the exam objectives before submitting my post (duh!).


BTW: www.learntosubnet.com is back up -- great find, Thanks!

 

No worries, I know that site helped me heaps

 

Managed to almost finish Mike Meyers's section yesterday, got a bit lost at the end as to how to re-form the binary to form something like 255.255.255.192. But I can give the purpose of subnetting, so i will give it another go, but i think the 1st half of MM book surley covers the objective from comptia?

 

Steve,

As a final check before sitting the exam, I would download the objectives and go through them one by one. Make sure you are comfortable with all of them.

Si

 

Cheers Si - I will do, im quite far away at the moment. What I meant was that the only bit about subnetting is to "know the purpose of subnetting", and MM chapter on this takes us into a lot of detail, so what ive learnt above should hopefully be ok for the exam!

 

Steve,

From what I can remember i didn't get any subnetting questions on the A+, so brace yourself for the N+

Best of luck.

Si

 

The only thing about subnetting you will be aked, if any is that you know what an internnetwork is, and that it needs a subnet, and thats it, so basicaly you will need to understand that Internetworks require subnetting, you will only be asked why you subnet, but no questions on creating a subnet IP range.

 

I think you could get a bit more than that. I got a couple of questions along the lines of "what's the subnet mask for a class A address" as well as one where they give you a CIDR address and ask what the subnet mask is. I agree you're unikely to have to go into really in depth calculations, but it's worth being prepared for them just in case.

 

I also Got one around the CIDR area.....

 

All I had was a question on knowing the class range of subnet masks, subnetting is about stealing host bits from the IP address and creating new subnets, in a large, or enterprise network.