Please have a look at another example
http://glennavendano.com/C
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Subnetting CCNA Cisco Activity 6.7.4
Hi, I know it has been alot of these kind of questons, but didnt find any solution that I undestood :S. I am currently trying to understand the process and the thinking behind of these activitys.
Inside the activity I have this problem that need to be solved.
// They are giving me an adress and subnetmask for the host ip.
Problem 1
Host IP Address
172.30.1.33
Subnet Mask
255.255.255.0
// This is what they want me to answer.
Anyone that has time to explain to me how to think and get answer to these questons below?
I tried ask the teacher and reading Cisco material but I dont understand the thinking.
A explanation is prefered when helping me answering this queston. Really want to understand this :)
Number of Subnet Bits
Number of Subnets
Number of Host Bits per Subnet
Number of Usable Hosts per Subnet
Subnet Address for this IP Address
IP Address of First Host on this Subnet
IP Address of Last Host on this Subnet
Broadcast Address for this Subnet
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Answers
Also use this link to learn bit more
http://www.cisco.com/en/US
I had a simple answer ready, but after reading dosdet2's comment - there is nothing to add.
Like he said, it's math. Binary (or Boolean) algebra to be exact. When you get some experience in the subject, you'll be able to "calculate" subnets on-the-fly.
Also, a liitle heads-up, when they use a suffix to specify an address, like 192.168.0.1/24, the /24 points out how many network bits are used in the subnet (24 divided by 8 is 3, so the first three octets are used, leaving 8 host bits).
For the CCNA, you need to be VERY aware of how subnet networks are calculated, so do a lot of exercises on these things.
Good luck on the exam!
dosdet2:
Thanks for your answerl Dosdet2 :), It start to make cense now. Analyzing your answer^^
Greate answer ^^.
oO Interesting, so the subnet mask decides how many networks bits and host bits it is in a
ip-adress? Your example as you explained so well
""172.31.30.2
172.31.31.5 - not the same subnet - the 1st 3 octets do not match.""
You do mean with?: doesnt match with the current subnet mask? (255.255.255.0)
Did you use ANDing? (1 and 0 = 0, 1 and 1 =0). Didnt understand that ^^
""Given the first subnet above: 10.10.1.25 = host IP address.
What is the very first and the very last address possible to still stay in this subnet?
This should be easy given that the host portion of this IP address is the entire 4 octet.""
Is it becouse its an A-class adress or is it becouse the subnetmask says it?
""Octets go from 0 to 255 so the first address possible would be: 10.10.1.0 and the very last possible address would be 10.10.1.255 - correct?""
You mean network adress right? or? Trying to follow you hehe.
""172.31.30.2
172.31.31.5 - not the same subnet - the 1st 3 octets do not match.""
Bit will 172.31.30.255 match? I see that we are using a subnetmask
with 24bits of networks, that means 172.31.30 is static while only
the last octet can be host bits or borrowed to create new subnets? (my head spins..lol)
""Now with that said, the subnet needs a name to identify it, so the subnet steals the first possible address of every subnet to be it's identifying name. By doing this, it makes that first address un-useable by a host. This identifying address is called the subnet address.""
I dont understand this? You mean a network bit in the last octet or DNS? or?
Ex; xxxxxxx.xxxxxxxx.xxxxxxxx.
B= BroadCast?
0= Networkbit?
""So out of every subnet""
What do you mean with every subet? You talking about
different subnet mask in different adresses or talking
subnetting further then the default subnet mask?
""Does this help?""
Hehe trust me i does :). I am currently just trying to understand more from your answer.
Everthing weill be so cleared if explanation behind each process, take information
from the subnetmask, or take from ip host adress, using ANDing method to determinate
network adress, I just dont understand why the activity call it Subnet adress?.
>>jayasanker<<
Thanks :); its good to see if I have done correct. :) I have tried to read these
kind of information, dont really understand the thinking behind it.
>>Andy_Quaeyhaegens:<<
Cool :), yes I follow you there, understand.:)
Btw thanks, I will need it :).
172.31.30.2 - 172.31.31.5 - not the same subnet - the 1st 3 octets do not match.
You do mean with?: doesnt match with the current subnet mask? (255.255.255.0)
Did you use ANDing? (1 and 0 = 0, 1 and 1 =0). Didnt understand that ^^
*** Actually they don't match each other (the subnet bits), but using the subnet mask to determine which are subnet bits and must match. The Mask determines which bits are subnet bits and which are host bits. If the subnet bits match (using ANDing) then they are in the same network. If I changed the Mask to 255.255.0.0 - then they would be in the same network because I reduced the number of subnet bits that must match. Try ANDing these two addresses and apply the 2 masks and I think it will become more clear.
Given the first subnet above: 10.10.1.25 = host IP address.
What is the very first and the very last address possible to still stay in this subnet?
This should be easy given that the host portion of this IP address is the entire 4 octet.
Is it becouse its an A-class adress or is it becouse the subnetmask says it?
*** It IS a class A address, but we have applied a more restrictive mask. A standard class A address (255.0.0.0) allows for fewer subnets and more hosts. Since we do not want 16 million hosts on each subnet, we use a more restrictive mask to get more subnets and fewer hosts per subnet. Because our Mask (255.255.255.0) says that all 3 of the first octets will be subnet bits, that leaves 256 possible IP addresses for each subnet. Minus 2 (one for subnet name [first] and one for broadcast [last]) that leaves 254 addresses available for Hosts in each subnet.
""Octets go from 0 to 255 so the first address possible would be: 10.10.1.0 and the very last possible address would be 10.10.1.255 - correct?""
You mean network adress right? or? Trying to follow you hehe.
**** Here I was talking about just IP address numbers. The very first possible IP address with the subnet bits (first 3 octets) matching and the very last one that also matches. The next statement was showing that the first and last IP numbers in each subnet are reserved and not available to assign to hosts.
172.31.30.2 172.31.31.5 - not the same subnet - the 1st 3 octets do not match.""
Bit will 172.31.30.255 match? I see that we are using a subnetmask
with 24bits of networks, that means 172.31.30 is static while only
the last octet can be host bits or borrowed to create new subnets? (my head spins..lol)
**** Static is probably not the best word to use for this, but I see how it kind of fits. The IPs listed are Class B so the default mask is 255.255.0.0. We have applied the mask 255.255.255.0 so we have borrowed 8 bits from the host side to use on the subnet side. But by doing that, we have said that any different numbers in the 3rd octet will be on a different subnet. That is why the two Ip addresses listed are in different subnets.
""Now with that said, the subnet needs a name to identify it, so the subnet steals the first possible address of every subnet to be it's identifying name. By doing this, it makes that first address un-useable by a host. This identifying address is called the subnet address.""
I dont understand this? You mean a network bit in the last octet or DNS? or?
Ex; xxxxxxx.xxxxxxxx.xxxxxxxx.
B= BroadCast?
0= Networkbit?
**** First Item - DNS is above layer3. It only matches english type names (URLs) to IP numbers. It has NO place of effect in defining subnets.
**** Second - I said the first possible address, not the last host bit. For example, the subnet that contains the address 10.10.10.10 with a mask of 255.255.255.0. According to this mask, the first 3 octets are subnet bits and cannot be touched. See binary representation below.
00001010 . 00001010 . 00001010 . 00001010 = 10.10.10.10
subnet bits to here ---------------> | - so this subnet must contain 10.10.10 for the first 3 octets in order to be in the sane subnet as 10.10.10.10.
So the first possible address in this subnet is:
00001010 . 00001010 . 00001010 . 00000000 - The same first 3 octets, with the lowest possible number in the 4th octet - or 10.10.10.0
Also the last possible address would be:
00001010 . 00001010 . 00001010 . 1111111 - The same first 3 octets, with the highest possible number in the 4th Octet - or 10.10.10.255
Remember if things don't make sense, convert them to binary. That is the secret to understanding subnets. There is no way to understand this without going to binary.
""So out of every subnet""
What do you mean with every subet? You talking about
different subnet mask in different adresses or talking
subnetting further then the default subnet mask?
**** The mask defines where the subnets starts and ends. - remember binary. I can tell by your questions that you are trying to understand this without going to binary. I can't stress enough that binary is key to understanding this. If you aren't willing to go to binary, I suggest choosing another field because you will be fighting to understand this all the way.
**** The subnet Mask defines how many bits are subnet bits and how many are host bits. With the
mask 255.255.255.0, you have the capability or over 16 million subnets. Example using this mask.
Subnet bits = | < --------------------------
10.10.0.0 = one subnet (00001010 . 00001010 . 00000000 . 00000000
10.10.1.0 = one subnet (00001010 . 00001010 . 00000001 . 00000000
10.10.2.0 = one subnet (00001010 . 00001010 . 00000010 . 00000000
10.10.3.0 = one subnet (00001010 . 00001010 . 00000011 . 00000000
10.10.4.0 = one subnet (00001010 . 00001010 . 00000100 . 00000000
And so on....
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by: dosdet2Posted on 2009-08-30 at 18:17:52ID: 25220135
OK Chugarah,
I'm going to attempt this. First of all, Subnetting, masks, host bits, Network bits - the whole thing drills down to binary. Once in binary, it should easily make sense. I'll assume you know how to convert from decimal to binary and back. If you don't know ho to do this, you need to learn this first or you will always be confused on this issue.
I'm using the tern `network bits' which is the same thing as `subnet bits' as far as this explanation goes.
Lets start with the Mask. That is where all the magic comes from.
Convert the mask to binary:
255 . 255 . 255 . 0 equals...
11111111 . 11111111 . 11111111 . 00000000
In the mask, the network bits are all the ones starting from the left - working right until you hit your 1st zero. The rest are host bits.
Here you have 24 network bits, leaving 8 host bits. Referred to as /24 in IP notation
For 2 IP addresses to be in the same subnet, they MUST match all the network bits.
Using the mask you used above, which is a standard Class C mask, is teally very easy.
The network bits split from the Host bits at the octet split, which means that you know that to be in the same subnet, 2 IP numbers must have the same values in the first 3 octets. So with this mask:
10.10.1.25
10.10.1.255 - same subnet
192.168.55.0
192.168.55.199 - same subnet
172.31.30.2
172.31.31.5 - not the same subnet - the 1st 3 octets do not match.
Given the first subnet above: 10.10.1.25 = host IP address.
What is the very first and the very last address possible to still stay in this subnet?
This should be easy given that the host portion of this IP address is the entire 4 octet.
Octets go from 0 to 255 so the first address possible would be: 10.10.1.0 and the very last possible address would be 10.10.1.255 - correct?
So the number of total possible addresses would be 255 + 1 (when you count the zero address).
So 256 total possible addresses can be in the above subnet.
Now with that said, the subnet needs a name to identify it, so the subnet steals the first possible address of every subnet to be it's identifying name. By doing this, it makes that first address un-useable by a host. This identifying address is called the subnet address.
Also, every subnet needs a broadcast IP to enable Hosts & routers to find their peers and discovers who is on their subnet. You can't communicate with someone on your subnet unless you can find them. The last possible address in the subnet is also stolen to act as this broadcast address.
So out of every subnet, the first and last possible addresses are stolen bu the subnet and cannot be used for hosts.
So the first HOST address on the subnet is the 2nd possible subnet IP, and the last HOST address on the subnet is the 2nd to the last possible subnet IP.
Does this help?