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I am trying to plan out a network with the following requirements and am having problems trying to complete a subnet plan for my network.

The network consists of 3 routers that connect 3 different buildings together. I need to subnet the entire network into 4 smaller networks using the - address range.

  • Router R1 hosts the Facilities network where 60 hosts are required.
  • Router R2 hosts the Business Ops network, which needs 100 hosts.
  • Router R3 hosts two switches:
  • 1.) Personnel, and they have 25 hosts.
  • 2.) IT, which has 25 hosts.
  • Each router-to-router connection requires 2 hosts via serial.

My initial thought was to use the following; however, this leaves me with an issue and I believe that it has to do with the /25 segment using/wasting too many addresses.

  • /26 for 62 hosts for Facilities
  • /25 for 126 hosts for Business Ops
  • /26 for 62 hosts for Personnel and IT
  • /29 for 8 hosts for each router-to-router connection

I am not sure why I cannot make them all fit within the required IP allocation and believe that I am missing something simple.

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Why do you want to fit everything into a single /24? You get much nicer addresses by using a /24 for each subnet. – Nic Oct 27 '11 at 5:36
Further to Nic's comment: you also get scalability. Sizing to exactly what you have today means a redesign next week. There's 16.8 million addresses in, and another million in The world may be running out of IPv4 addresses, but your internal network is not. – Shane Madden Oct 27 '11 at 16:57
up vote 7 down vote accepted

No matter what you do, the following addresses will be consumed minimum:

Facilities:  64
BusOps:     128
Personnel:   32
IT:          32
Toal        256

This leaves no room for the router-to-router connections, assuming you're trying to number them all inside the same /24.

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You are correct - I am trying to number them all within the original /24. Is there a different way to lay out the subnets? Am I missing something? – John Oct 27 '11 at 5:27
Is there a better way to combine some of these to achieve the same desired results? – John Oct 27 '11 at 5:28
I would suggest numbering the router-to-router links from some other block. Actually, since 10.x.y.z is so big, I'd suggest just giving each of the four networks its own /24. – David Schwartz Oct 27 '11 at 6:06

This is actually possible, if your routers do not use a PtP link (which is normal). You can use the subnets laid out by David, and put the 3 routers into each other's subnets.

As others have stated, this is still a bad idea and you should be using at least full /24s for each subnet since there's already a good number of hosts in each and growth is inevitable. Plus it's much easier for a human to read the IPs and know where the traffic is coming from.

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Sounds like a homework assignment.

It looks like you are trying to put all of the router links in the same subnet. That will not work; how would a router know which router to send a packet to? Each router to router subnet must be a different segment. Ideally router to router would be a /30 (.252) giving a block size of 4, one broadcast one subnet, two hosts.

Now you can start at 255.255.255.x depending on what block size you need and work your way through the list.

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