Subnet mask question

susuandme

On some of the subnetting questions mentioned earlier, I have
also seen some questions where the diagram says:


IP address: 10.24.12.1
subnet 255.255..0.0 or 255.255.255.0


How can an IP address that is starting with a "10" have an network ID with 2 or 3 octets long, I thought it was 255.0.0.0

Is this a "private" IP address, if so what is the correct subnet mask for a "10" private id address. thankyou. Rick

dynamik

Networks can be divided by extending the subnet mask. If you weren't allowed to do this, an enormous amount of addresses would be wasted. You'll learn more about how this works in 291 or ccent/ccna.

SamCarter

If the subnet mask were always the same (depending on the IP) then you wouldn't need it. What you are talking about is the default subnet mask for each class:

A: 255.0.0.0
B: 255.255.0.0
C: 255.255.255.0

But you can always deviate from that by using a different subnet mask. The 255's indicate which part(s) of the IP belong to the network and the 0's show which part is for hosts.

You have to be able to recognize problems with an existing setup but I don't believe it is part of this test's objectives to be able to actually do subnetting.

http://www.tcpipguide.com/free/t_IPDefaultSubnetMasksForAddressClassesABandC.htm

Daniel333

http://www.learntosubnet.com/ really helped me learn it.

You won't be tested on that on the 70-270 exam, so don't get stressed about it. BUT it's knowledge you must have before starting the 70-290 etc.

what are you goals?

susuandme

My goal is to pass the mcp, and ultimately get an MCSA or mcse

I still do not follow you as far as the 10.12.2.1 why isn't this subnet
mask 255.0.0.0. its important, because some of the subnetting quesdtions
on the mcp exam, give a diagram, where the subnet mask is wrong for the "10"
type ip address, and the question asks you what is wrong and why are they unable to communmicate .

a. gateway. b. subnet mask c. dhcp, etc. etc.

So what am I supposed to answer if the subnet mask doesn't look right in this example. I may answer the question wrong, because your saying now
that the subnet mask can be anything. I'm totally confused now.

But I will check your link now. Thankyou, Ric

dynamik

I see what you're getting at. For this exam, you're just going to want to make sure that the subnet masks are the same. Subnetting is a relatively complicated process (at least at this level) that is used to conserve addresses and segregate networks. If you're given a scenario where it lists a few machines along with their IPs, subnet masks, gateways, etc., you're going to want to make sure the subnet masks are the same for the devices that are trying to communicate. Essentially, different subnet masks means the machines are on different networks, and will be unable to communicate.

(I know this is a gross oversimplification of subnetting, but I'm trying to put it in the context of the 70-270 exam)

susuandme

Okay, I understand, the subnet masks should be the same, because
only subnet masks that are the same, with the same network i.d. can communicate on the same physical segment.

Is it true then, that in order for different subnet masks to communicate, then they will need a "gateway: or routeer" ?

dynamik

Correct. Just remember that the router will need one interface on each subnet. Don't just assume that everything is fine because a router is involved. You will need to look at the IP and subnet mask of the router interfaces as well. You need to treat that just like another computer on the network. If the subnet mask is wrong on the router interface, the machine won't be able to reach it, and it will consequently not be able to reach machines on other subnets.

hoomi_mcse

it is call subnetting . here you add some of the 0 bits of host part to the bits of network . it means you change 0 bits to 1 . for instance when you have a ip address like this :
10.0.0.1 with subnet mast of 255.255.255.0 . you have added 16 bits of host to network part so it has become like a class c subnet . thus 10.0.0.1 and 10.0.0.26 with subnet mask of 255.255.255.0 are in a same network in this example. it can be more complex when subnet mask is for instance 255.255.224.0 or 255.192.0.0 . but as other mention ed in 70-270 you wont face such questions.

Daniel333

susuandme wrote:

My goal is to pass the mcp, and ultimately get an MCSA or mcse

Since the 70-270 probably won't test you on this, go ahead and move on. There is no real way we can teach you to subnet in a forum, it's pure math and the best way is just sit down with a text dedicated to it with pen and paper for a dozen hours and then go into a lab for a dozen more.

I asked you what you are goals are because subnetting doesn't go away, ever. It's just going to get crazier.

Complete your 70-270 and 70-620 and I would like to see you start on your CCENT. This is a somewhat easy certification from Cisco which will spend a great deal of time hammering in Subnetting, supernetting, addressing and talk a lot about the networking equipment you need to have a solid understanding of before you should proceed to the MCSA/MCSE's 70-290/70-291 etc...

Since any job you take will deal with routers, switches you can rest assured that the time spent studying for the CCENT will not be wasted.

Lastly if you still find that after you have spend a few days on www.learntosubnet.com I have found the videos at www.vtc.com be be pretty good. They have a whole section dedicated to TCP/IP. So you will get a more practical view of it there, but complete the learntosubnet one first.

Best of luck to you,

susuandme

thankyou, and thankyou for having helpful kind people like you on this planet,
sometimes I get disallusioned then get reassured again from kind wonderful people.

I did get a "Router Interface " question, on the diagram, there were two
subnets, subnet 1 and subnet 2. a workstation 1 was trying to communicate with subnet 2 but could not. In this example, if the router has two interfaces,
"WHICH INTERFACE" should I make sure is correct. Should I make sure the router interface is the same on 'MY SIDE OF THE SUBNET" ?

fOR EXAMPLE. if my I.P. was:

MY I.IP 10.12.15.7
ROUTER - 10.12.15.1 AND 10.12.14.1

Would the answer in this case be that the 2nd router Interface is different
and so that is why my computer could not connect ?? Rick

dynamik

Yes, you will need the interface to be on the same subnet. If it's not, it won't be able to communicate with it, and as a result, will not be able to forward packets to other networks.

SamCarter

MY I.IP 10.12.15.7
ROUTER - 10.12.15.1 AND 10.12.14.1

In this case, if they were using a subnet mask of 255.255.255.0, then the easiest thing is to look at the first three sets of numbers to figure out which address is in your network. So you should choose the interface on 10.12.15.1 because it is in the same network as your IP.

If they gave you a subnet mask of 255.255.0.0, then the answer could be either one (10.12.x.x).

It gets confusing later on, but that is really all you need for now. I would suggest that you not get distracted by all this subnetting stuff and just focus on the test and hand now. After you tackle that, you can approach it fresh and really understand it. I studied one book's take on subnetting at first but it didn't "click" at all and ended up looking at a bunch of different sources (and forum posts, actually) before finding an explanation that helped me "get it".

TravR1

You'll do just fine, don't stress it. I have been studying subnetting on and off for over a year and I still don't totally understand it... not completely anyway. Like mentioned above.. you will be thinking on subnetting for awhile to come. It's like those designs that you stare at long enough then pops out a picture... well if you study it for long enough eventually one day it's just going to come together in your head.

I used all those recources listed above.. I also searched subnetting in youtube and found a few great videos posted by people trying to help guys just like you. Personally I liked the videos created by 'vegasrage' ... his approach to teaching subnetting made a lot more sense to me than the other approach that is used often.

susuandme

thankyou all, if I knew what I know now about router interfaces I might have
passed the exam, I got a 677 last time, also i was destroyed by
the permission questions on the exam, because they were lengthy and
confusing. some other questions on the exam test your memory on "minor" details about how long a password can be, or

A Person cannot see the "Media Pool" icon in Computer magement, should she use:
A. Simple View
B. Full View


These are the type of questions that worry me, because I will have to learn every single minute detail on some questions. which means memorizing a ton of info. Is there a way to accomplish this without having to understand every single "link" "tab" profile box" etc. If so its going to be another months work.
Thanks

dynamik

susuandme wrote:

thankyou all, if I knew what I know now about router interfaces I might have
passed the exam, I got a 677 last time, also i was destroyed by
the permission questions on the exam, because they were lengthy and
confusing.

Well, it seems like you still have a few weak areas, so it's probably good you didn't just squeak by and move on to something else. If you try to go too fast without building a solid foundation, you're just going to have more trouble later on.

susuandme wrote:

If so its going to be another months work.

While I agree some questions like that are a bit ridiculous, we all have to put our time in. Some of us have spent years tinkering around with this stuff before we even started studying for the exam. If you're just starting out, you're going to have to make up for that lack of experience. It seems like a lot, but it will come with time.

hoomi_mcse

oh unlucky with your score. I think if you work on permission and have some good practice on it then you can pass the exam. with your score it seems you're prepared enough for other parts of exam so I suggest you work on you weakness and be sure you can pass it next time .

susuandme

thankyou, I wil try,


I came across another subnetting problem this one involved a DNS
SERVER.

The DNS Server was on a differetnt Network Segment.

There was Segment 1 andSegment 2 separated by a router
in the middle., called a gateway of course.

On Segment 1. Client A could not communicate with its Data Server B
on the same the same segment.

I'll skip to the Answer. The answer was that the DNS server which
was on a 'DIFFERENT SEGMENT WITH A DIFFERENT NETWORK I.D. HAD
TO HAVE ITS I.P. CHANGED TO THAT OF THE SAME NETWORK AS
AS SEGMENT 1. EVEN THOGH IT WAS ON A DIFFERENT SEGMENT.

Why is that so ? I thought only gateways had to have similar subnet
masks if on the same net. Why was it that even though the DNS
SERVER was on a completely different segment, the subnet mask, and
Network I.D. had to be changed to that of coinciding with the other segment.
Is this an Exception, are there othe exceptions like DHCP servers,also, etc.

hoomi_mcse

all computers in a segment must have a same network ID and different Host id no matter it's a client, Domain Controller, DNS,DHCP, Router inerface etc. if any of these computers have different Network ID they can't communicate with any of the other computer in that segment.

skrpune

was there more info in the question that you didn't include in your post?? Were IP addresses/subnet masks included or a network diagram, etc.? I'm a relative beginner with subnetting & IP addressing so maybe it's my inexperience, but I can't figure out why what you show as the answer IS the answer without a little more info, like what the actual question is or what the other answer choices are.

And by the way, was the router called a "gateway" in the example/question, or are you assuming that it's a gateway?

susuandme

I thought all routers are gateways,

but yes the DNS server was on the other side of the router, the dns
server had a different network id, and the answer was that odd, If I can
I will try to find the question again if I can in my book, thankyou.

can you upload diagrams on this board, that would make it simpler.

dynamik

susuandme wrote:

can you upload diagrams on this board, that would make it simpler.

The easiest way would probably be to create an account somewhere like PhotoBucket, ImageShack, etc., upload the images there, and then link to them there.

skrpune

Not sure how familiar you are with the OSI model, but in general routers are on layer 3 (network layer) and gateways are generally on layer 4 (transport layer) & above, so they operate differently. Some routers can be configured to act as gateways, but so can computers & other devices, and not all routers are gateways.