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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 10.10.1.0 - 10.10.1.255 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 10.0.0.0/8, and another million in 172.16.0.0/12. The world may be running out of IPv4 addresses, but your internal network is not. –  Shane Madden Oct 27 '11 at 16:57
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3 Answers

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
2  
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
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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. 10.10.10.0 255.255.255.252 10.10.10.4 255.255.255.252 10.10.10.8 255.255.255.252 10.10.10.12 252.252.252.252

Now you can start at 10.10.10.16 255.255.255.x depending on what block size you need and work your way through the list.

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