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I have a quick query about using VLSM. I need to adopt a optimized addressing scheme making use of VLSM, I have 3 networks in total connected via two different routers, router 1 has a network of 300 hosts and another network of 25 hosts. Router 1 is connected to router 2 and that has an additional 82 hosts. Would it be possible to do this scheme over two IP addresses?

Lets say my starting IP is 182.20.1.0, how would this work?

mitchnufc
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  • Smells like homework to me? does it have to be IP v4? – Chopper3 Apr 11 '12 at 14:19
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    yeah, homework I think. really need to do it yourself to get the concepts right. please attempt it and then others may be able to critique it. really basic. – johnshen64 Apr 11 '12 at 14:27

1 Answers1

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With VLSM you start with the biggest number of hosts:

300 hosts means you need 302 IP's. 302 IP's means you will be needing a /23 because you want to start at 182.20.1.0, you will lose some IP's because 300 hosts will not fit into 182.20.1.x:

  • So the first IP will be 182.20.2.1
  • The last IP will be 182.20.3.254
  • network address will be 182.20.2.0
  • broadcast address wil be 182.20.3.254
  • The subnetmask will be 255.255.254.0

Then we take the next biggest amount which is 82 (so the smallest available subnet is /25):

  • The first IP will be 182.20.4.1
  • The last IP will 182.20.4.126
  • network address will be 182.20.4.0
  • the broadcast address wil be 182.20.4.127
  • The subnetmask will be 255.255.255.128

Then we still have our network with 25 hosts left (27 IP's, the next subnet available that can handle this much hosts is /27):

  • The first IP will be 182.20.4.129
  • The last IP will be 182.20.4.158
  • The network address will be 182.20.4.128
  • The broadcast address will be 182.20.4.159
  • The subnetmask will be 255.255.255.224

Mind though that in reality you can make the last 2 networks fit into a /24, this allows some breathing space if you ever need to add more hosts.

Lucas Kauffman
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  • Thanks for the help. I understand the full concept of VLSM just was wondering if there was anyway to limit a network to 300 hosts, which you confirmed isn't possible as the I could either have 510 hosts or spilt the networks down into a 255 and a 64 network. – mitchnufc Apr 11 '12 at 15:07
  • no problem, you can always accept the answer if you think it is good enough :) – Lucas Kauffman Apr 11 '12 at 15:24