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IP subnetting

Posted on 2008-06-14
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Last Modified: 2012-05-05
often get stuck when it comes to  a subnet mask that has more subnets than the default subnets for an IP CLass.

I found some examples with solutions but still confusing:
example : 10.16.3.65/23  it says that This is class A address and it also says that host 10.16.3.65 is in the 2.0 subnet, the broadcast address for the 2.0 is 3.255, the valid host adresses are 2.1 through 3.254.

I still can't understand when it come to x.x  as in 2.0 or 3.254.

Can someone explain to me clearly how does this work?
thanks,
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Question by:jskfan
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by:cybrexus
cybrexus earned 800 total points
ID: 21784731
ok so there are 5 classes of ip addresses, you can read more about subnetting and classes here http://en.wikipedia.org/wiki/IPv4_subnetting_reference

the classes do not depend on your subnet e.g. 10.16.2.33 is of same class as 10.0.0.1 - Class A
what determines the class is the range of ip addresses descibed in the document above

when you subnet, you don't change the address class, you just divide you ip address in smaller ranges, this allows you to assign different portions of the same ip address to different networks

here is a good subnetting calculator http://www.subnet-calculator.com
try it out, put in different ranges and ips, hopefully it will help you to understand

why for subnet 2.0 you have broadcast 3.255? again because you have a mask of 23 which means that you are taking 1 bit from the 3rd portion of your ip address, therefore the range of your ip addresses for this subnet is 10.160.2.1 - 10.160.3.254
if you would have picked 24 as mask then your range would have been 10.160.2.1 - 10.160.2.254

hope this helps
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by:fhmc
ID: 21785483
for internal purposes (actually, for ALL purposes) the default mask for any IP class is irrellevant w/ regard to customized subnetting.

when I explain how subnetting works to my techs, I often break it down like this:

most folks are familiar w/ subnet masks in the following format 255.255.255.x etc.
some folks are familiar w/ subnet masks in the following format /24
many folks dont understand subnet masks in the following format 11111111.11111111.11111111.0

the way I think of subnets and the way I explain subnetting is in the form I described in line 3 abotve.

the 10.x.x.x IP range is "by default on the 11111111.0.0.0 subnet

you can override that default mask to any mask you like.
say you want to use the 10.x.x.x network and you want 4 subnets defined that will accommodate UP TO 1022 hosts each, with room for growth.

every subnet loses two (actually 3 addresses)
1 = base (or network address)
1 = broadcast
1 = a router address

1022 hosts is closest to, but less than the folllowing binary number "1024"
1024 in binary is 9 bits
a subnet mask is comprised of 32 bits
11111111.11111111.111111111.1111111 - 9 bits or is 11111111.11111111.11111110.00000000
or 255.255.128.0, or /23  (32-9 = 23)
starting from the ground up, your subnets will be defined as follows
10.0.0.0/23  network address = 10.0.0.0 broadcast = 10.0.1.255
10.0.2.0/23 network address = 10.0.2.0 broadcast = 10.0.3.255
etc.

not sure if this is the help or answer you were looking for... either way, I hope you found it to be helpful.
errrr...  and I hope I got it all right.  I just woke up and my brain is still a little fuzzy.. heheh

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by:jskfan
ID: 21786193
Here is the question from CCNA book
Which two statements describe the IP address 10.16.3.65/23? (Choose two.)

Here is the answer from CCNA book:

The mask 255.255.254.0 (/23) used with a Class A address means that there are 15 subnet bits and 9 host bits. The block size in the third octet is 2 (256  254). So this makes the subnets in the interesting octet 0, 2, 4, 6, etc., all the way to 254. The host 10.16.3.65 is in the 2.0 subnet. The next subnet is 4.0, so the broadcast address for the 2.0 subnet is 3.255. The valid host addresses are 2.1 through 3.254.

what do they mean by  The host 10.16.3.65 is in the 2.0 subnet. The next subnet is 4.0 ??
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by:fhmc
ID: 21786212
I believe my last post should explain this.

10.16.3.x is in on the 10.16.2.0 network
e.g.  10.16.2.0 through 10.16.3.255
the next available subnet is 10.16.4.0 (which will range through 10.16.5.255)

also, I'm a bit confused about the 15 subnet bits and 9 host bits.
shouldn't that be 23 subnet bits and 9 host bits?
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Author Comment

by:jskfan
ID: 21787094
I guess they mean if we had 10.0.0.0  it should at least have 255.0.0.0 which is 8 bits on by default
in this case 10.16.3.65 /23  has 15 more bits on other than the 8 default ones which makes it 23 bits on for subnets and 9 bits left for hosts.

where I am confused is the expression:
<<<The host 10.16.3.65 is in the 2.0 subnet. The next subnet is 4.0, so the broadcast address for the 2.0 subnet is 3.255. The valid host addresses are 2.1 through 3.254.>>>
why the 65 is ignored?
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by:
fhmc earned 1200 total points
ID: 21787105
.65 isn't ignored.  10.16.3.65 is merely in the range between 10.16.2.0 -> 10.16.3.255 so it's included in the 2.0 subnet

any IP address in that range is a participant in the 2.0 subnet.
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