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class B subnetting

Posted on 2007-11-17
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If we have a class B network x.y.0.0/20 for example.
How many subnets we have ? 16 or 14?
That means x.y.0.0 and x.y.240.0 are valid subnets or must be reserved to notify the hole
net(x.y.0.0) and  broadcast to all subnets (x.y.240.0) ?
Jim
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Question by:dimi67
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Expert Comment

by:beckman55
ID: 20304926
With the subnet you have you have an incrementing factor of 16.   So your ip address' will be as follows:  with a subnet mask of /20

x.y.0.0
x.y.16.0
x.y.32.0
x.y.48.0
etc

x.y.0.1-x.y.15.254

x.y.16.1-etc

I hope that answers your question.  If not please give me some more information
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Accepted Solution

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pavaneeshkumar earned 2000 total points
ID: 20304984
The table below summarizes all of the Class B subnet masks and the number of
networks and hosts.
Subnet Mask 	# of Unique Valid Networks	# of Hosts on Each Network
255.255.0.0 	1—standard subnet mask		65,534
255.255.128.0 	Invalid—only 1 bit for subnet portion
255.255.192.0 	2 				16,382
255.255.224.0 	6 				8,190
255.255.240.0 	14 				4,094
255.255.248.0 	30 				2,046
255.255.252.0 	62 				1,022
255.255.254.0 	126 				510
255.255.255.0 	254 				254
255.255.255.128 510 				126
255.255.255.192 1,022 				62
255.255.255.224 2,046 				30
255.255.255.240 4,094 				14
255.255.255.248 8,190 				6
255.255.255.252 16,382 				2
255.255.255.254 Invalid—only 1 bit left for host portion 
255.255.255.255 Invalid—no bits left for host portion

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Author Comment

by:dimi67
ID: 20305010
the two answers are different...wich is the correct?? 14 or 16??
kumar please be more specific..why x.y.0.0 and x.y.240.0 are not valid subnets?
Thanks in advance...
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Expert Comment

by:beckman55
ID: 20305027
valid subnets can only end in the following 0,128,192,224,240,248,252,254,255

So 255.255.0.0 and 255.255.240.0 are both valid subnets.  It depends on how your networking them and how you want to use them.  His chart is correct you will only get 14 networks with a 255.255.240.0 subnet.    My half that I explained shows how those 14 networks are broken up in the x.y.240.0 subnet.  x.y.0.0,x.y.16.0, 32, 48, 64, etc 14 times till you max that subnet.

Give us a little more info on what your trying to accomplish if that doesn't answer your question
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Expert Comment

by:beckman55
ID: 20305034
maybe just a little more to clarify:

IP address example:  

10.10.16.1 - ip
255.255.240.0 - subnet

10.10.32.1.
255.255.240.0
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Author Comment

by:dimi67
ID: 20305162
in the sequence of x.y.0.0, x.y.16.0 ...x.y.240.0 we have 16 subnets and not 14...the question is : is 16.0 and 240.0 valid subnets?
thanks
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Expert Comment

by:pavaneeshkumar
ID: 20305232
255.255.26.0 is an invalid subnet
and
255.255.240.0 is a valid sunbet for class B.

please have a look on the table posted previously.
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Expert Comment

by:pavaneeshkumar
ID: 20305237
also there are only 14 valid subnet under Class B network not 16.
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