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I have the following subnets:

10.1.1.80  255.255.255.240
10.1.1.96  255.255.255.240
10.1.1.112 255.255.255.240
10.1.1.128 255.255.255.192

How can I combine all these subnets to form one single subnet so that the first usable address in the subnet is 10.1.1.81 and the last usable one is 10.1.1.190?

  • I've been trying to formulate an answer but I cant get it readable enough. But remember that netmasks are in fact binary and need to be of base 2. This means a subnet must be of size 1/2^n of the original adress space and the number if IPs in a subnet always is 2^m. You are asking for a 111 adress subnet but 111 is not base 2. (2^7 = 128) – Peter Jul 9 '15 at 10:16
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    This looks suspiciously like homework – Ron Trunk Jul 9 '15 at 11:28
  • Welcome to NE, we hope you will both contribute to and learn from this community. This question comes across sounding suspiciously like a homework question, which is off topic here. If it is not school work, please edit your question to provide more context about why you are asking and it will automatically start a reopen vote. You may find our Question Checklist helpful to improve your question. – YLearn Jul 9 '15 at 19:56
  • Hi. I am not in school. I am a Network Professional and CCNP certified. I can understand your point of view that this may seem as ridiculously simple question. However this was a need of one of my customers. Currently he has servers in 10.1.1.128 / 28 subnet and wants to enlarge the subnet by using addresses starting from 10.1.1.81. I told him this was not possible but he is totally convinced that this is possible. So I thought probably I am missing something really fundamental and so I posted this here. And if this is a "Networking" forum I don't understand why such questions are a problem !? – Thorsten Dahl Jul 10 '15 at 11:37
  • Additional Sidenote - The crazy customer also wants me to consult Cisco about subnetting ! Any ideas how to handle such people ? – Thorsten Dahl Jul 10 '15 at 11:38
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The lowest ip is 10.1.1.80, and the highest is 10.1.1.191 (from 10.1.1.128/26)

The smallest subnet (bigest mask) to cover both is 10.1.1.0/24

When you sub-dived the subnet 10.1.1.0/24, by increasing the mask to /25 you get two subnets, not one can covers all the subnets in the question.

10.1.1.0/25 (Range 10.1.1.0-127)

10.1.1.128/25 (Range 10.1.1.128-255)

The ranges you provided is not continuous, and the answer 10.1.1.0/24 covers them all and the network in between and at the start and end of the ranges.

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