Wendell odoms Subnetting not as good as i'd hoped.
pandimus
Posts: 651Member
in CCNA & CCENT
So i'm trying to get a refresher on my subnetting. Heck 216 was like 2 months ago.. Anyway, I am going through the book and I realize both books have a replica of the same chapter for subnetting.. (no biggy)..
But when i start reading it, i realize he makes it harder than it is. (IMHO). I think the best place's ive found to learn to subnet are.
www.Learntosubnet.com
http://www.mcsefreak.com/subnetting.htm
I guess i will review it again when less tired. But by what i could tell, he just didnt make it easy for me.. Perhaps someone else would like his style better. Or even someone who hasnt learned this stuff yet..
BTW, does anyone know if there is more to subnetting for cisco than there was for 216?
But when i start reading it, i realize he makes it harder than it is. (IMHO). I think the best place's ive found to learn to subnet are.
www.Learntosubnet.com
http://www.mcsefreak.com/subnetting.htm
I guess i will review it again when less tired. But by what i could tell, he just didnt make it easy for me.. Perhaps someone else would like his style better. Or even someone who hasnt learned this stuff yet..
BTW, does anyone know if there is more to subnetting for cisco than there was for 216?
Xinxing is the hairy one.
Comments
Now if someone can explain hex numbers to me in a way that makes sense...
Magnanimous as the ocean, persistent as time.
Now everything has a drawback--and I'm not blaming Todd for this--, but I just don't understand the VLSM part of the game. I do understand the basic concept, but I don't get this whole Block stuff and all. Just like how a file cabinet placed incorrectly onto the cabinet won't slide in; this VLSM isn't just sliding into place.
One of the things that confused me was the author took an example of a network and apparently gave it a wrong number of bock sizes.
This was the network .
Network ID:A--Mask:/27--Hosts:30
ID:B--Mask:/28--Hosts:10
ID:C--Mask:/28--Hosts:12
ID:D--Mask:/30--Hosts:2
ID:E--Mask:/30--Hosts:2
ID:F--Mask:/30--Hosts:2
ID:G--Mask/28--Hosts:12
ID:H--Mask:/26--Hosts:60
ID:I--Mask:/28--Hosts:14
ID:J--Mask:/30--Hosts:30
ID:K--Mask:/28--Hosts:8
Now does this network have the following block sizes (As specified on the book):
2 block sizes of 64, two of 32, four of 16, and three of 4,
OR
2 block szies of 64, one of 32, five of 16, and 3 of 4.
And what is the purpose of this block size in the first place
2lbs.
Magnanimous as the ocean, persistent as time.
I haven't seen VLSM questions put that way either. So I'd also be interested in the result.
While we wait for someone to answer that VLSM question (and elaborate)... here's one other problem you should consider in the exam.
The subnet mask /25 has only one bit available for subnetting. Now that bit is either 0 or 1--subnets 0 or 128 respectively. This subnet mask allows 2 subnets and 126 hosts per subnet. Now the strange part is that the author says "Cisco" says we shouldn't use this subnet mask (and consider it invalid in exams). At the same time, my router--and perhaps all routers-- support the ip-subnet zero command. To add to the confusion, the author says we can make use of this (and should) in real time production.
There's always a reason Cisco says something; so what's the moral behind this "do as we say, not as we do" story.
2lbs.
Magnanimous as the ocean, persistent as time.
He refers you to a website to learn how to change binary numbers into ip address's and vice versa.
But I think you should be able to do that in your head
You know... I think if someone's overqualified, he can't stay focused on one topic and explain it thoroughly. The guys a CCIE --keep that in mind. He probably knows 222 much
Magnanimous as the ocean, persistent as time.
and... of course, the question "Why?" still remains.
Magnanimous as the ocean, persistent as time.
Because we won't be smart enough to deal with that concept until CCNP? Another problem I have is that Cisco says for the CCNA standard ip access lists should be placed as close to the destination as possible and extended should be placed close to the source, yet I had 2 questions on the exam that had scenarios where it wasn't possible to place the standard access-list on the destination router.
haha..was that a "randomly-generated" answer. Well then we should get some CCNP guy to answer it. One thing I know is that big companies might just say something now because of something and stick to it even when things have changed. subnet-zero was probably a problem when they laid down that rule..... But then... exam contents aren't there for fun.... There has to be a real reason behind everything. (break; let's not make a fuss outa it. we'll pass if we do it Cisco's way... so why not?)
Access-lists.... haven't reached them yet.
2lbs.
Magnanimous as the ocean, persistent as time.
hahaha. Well a lot of people wish they could "wish" that very wish.
And something to laugh at: Cisco systems (probably) came from SanFransico Systems and the legal paper got crampled up--or teared-- en-route to the lawyers office only to reveal cisco Systems.
Magnanimous as the ocean, persistent as time.
I dont think it is a question of weather or not i can do that in my head, its the fact that his book is a learning tool, and if he only teach's you one part, and tells you to go to a website to learn the other part, he really isnt teaching you the whole thing. I honestly cant convert binary in my head yet, but i can write it on paper and have it down withing 30 seconds. Sofar i havent used binary except for my 216 test.
Needless to say, i think for a book to be a good book, they should teach you the whole concept.
No one could put it better. I totally agree.
I'm not saying this ostentatiously, but after reading Todd's explanation of binary to decimal conversion and subnetting; I can almost do it perfectly in my head. I hope I don't wake up tomorow morning with a blank memory
2lbs.
Magnanimous as the ocean, persistent as time.
i wish i could, my book refers me to the internet to learn how to convert binary to decimals.. I dont have access to the internet.. So now ive got to go to the library or buy another book. But if he would have included that...
Well is the internet link helpful at least....??
I thought people had to be on the net in order to post on forums
2lbs.
Magnanimous as the ocean, persistent as time.
http://www.goshen.edu/compsci/mis200/decbinary.htm
http://www.doit.ort.org/course/inforep/135.htm
http://www.ibilce.unesp.br/courseware/datas/numbers.htm#mark2.
just an example, not everyone in the world does have access.. I am sure people can get it regardless, and that is not really the point i am getting at.. The process is not hard and would have probably taken one more extra page.. At least...... in the bare minimum... put it in the intro book.....
128 64 32 16 8 4 2 1
and then putting the 1's and zero's in place and calculation the number?
"
As mentioned in RFC1878:
"For the sake of completeness within this memo, tables 2-1 and 2-2
illistrate some options for subnet/host partions within selected
block sizes using calculations which exclude all-zeros and all-ones
subnets [2]. Many vendors only support subnetting based upon this
premise. This practice is obsolete! Modern software will be able to
utilize all definable networks."
You 'may' run into questions on the exam where you don't have to subtract the two. But then it will be explicitely mentioned, i.e. "the entire network supports CIDR" or "all OSPF routers are configured with the ip-subnet zero command." In all other exam question scenarios you have to apply 'traditional subnetting'. But: this is correct.
A bit more, not all subnetting questions in the CCNA (and CCNP exams) focus just on subnetting. As routing is the primary topic, subnetting is an elementary part of several questions including sims.
Before Odom's Cisco Press book arrived on the scene, which seem to have become very popular, Todd Lammle always used to be 'the' author for CCNA books. He's been elected best tech author for several years in a row. If you want to use two books, I think combining Wendell's and Todd's books is the best way to go.
So we don't have to waste entire IP networks on small (sub-)networks. Without VLSM every subnet in an IP range would have to use the same subnetmask. Hence, if one subnet needs 64 hosts, you can't use the remaining hosts in a single subnet, you would have to use the block size 64. So VLSM allows an IP network, ie. class C network 192.168.10.0 to be divides into several subnets with a different block size. A common example, is the lab example we usually discuss in our forums, and TechLabs. Say you got one class C IP network for all three networks:
Without VLSM you would have to use the same subnetmask on network 2 as network 3, you could assign 192.168.10.0/26 to network 1, and 192.168.10.64/26 to network 2 and 192.168.10.128/26 to network 3(assuming it supports the ip subnet-zero). Leaving 192.168.10.192/26 unused. But more importantly, this would waste 62 addresses on network 2, because it only needs 2 (both ends of the serial link). With VLSM however, you could assign the mask 255.255.255.252 (/30) to network 2 and use other mask to use the remaining addresses for network 1 and 2.
VLSM is developed because of the shortage of IPv4 addresses.
Pandimus, maybe this older discussion we had can be of some use again:
www.techexams.net/forums/viewtopic.php?t=2935
I'm not entirely sure, but I think this has been discussed in earlier posts in the CCNA forum, please try a search. If you can't find anything please start a new topic.
They're not bad. But that info in a consice form should've been in the book. But still... you know this stuff already, so if it covers other chapters neatly, I don't think you should buy a new book.
How much did you pay for it??
2lbs.
Magnanimous as the ocean, persistent as time.
The reason i said that was so that people who dont know subnetting, might swerver towards a more thourough book.
I think i paid 40 bucks for the 2 book series, brand new, with a 10 dollar voucher and some decent practice questions. with a small version of boson router simulator.
Basicly, what he does is he makes you look at the byte as 2 nibbles. He then tells you that since the most important taks for binary to decimal conversion is subnetting, turning 1's and 0's off can only happen in a single file in the right most nibble (first 4 bits) starting from the right most bit.
i.e. 1111 would be 240. and 1011 (176) would not be used in subnetting masks. so essentially, the /25 to the /30 masks in class C would only include 1's and 0's switched on or off in a single file. The same will apply for the A and B classes. only the slash(/) notaion number would decrease.
the hard part is when you want to convert a binary-decimal for the host portion. since hosts will contain any numbers possible in the range of their bits, they won't have 0's and 1's switched on and off in a single file. But you won't be doing that headache anyways. host ID's are determined by multiplying 2 with the exponent of the number of bits you have left for the hosts after taking away the subnet bits and subtracting 2. And then valid subnet's would be determined by subtracting the subnet mask (in decimal) from 256. The result multiplied by itself untill you reach the subnet mask (excluding the mask) would be the available subnets. And the available hosts would be the numbers available between one subnet ID and it's broadcast address (which is always the next subnet ID minus 1).
2lbs.
Magnanimous as the ocean, persistent as time.
If so how far up do you go?
Instead, you should memorize only the roots up tol 8. So if we had a Class B Network with a /26 subnet mask, you won't find the root of 2^10, but rather the root of 2^8 (which is 256--and you should know all the roots from 1 to 8 by rote). You would then multiply the result with the root of the remaining bits (2 in our example which means 4). So that would be 256X4=1024.
This method is basicly reducing the number of roots you have to do in your head. But if you a mathematician, then you won't have to worry
2lbs.
Magnanimous as the ocean, persistent as time.
I was thinking about stopping there are reading the whole book over again untill I nail the previous stuff and have it dialed in. Do you think I should do that; or just move on??
How are you studying?? I know Wendell's book is 2 in one. Are you going to take the exams like that too?
2lbs.
Magnanimous as the ocean, persistent as time.