RE: Internet Building topology

With the information provided it seems the you should concentrate on logical connections first then work out the physical location of the equipment as a secondary matter. For all you know you may end up have one central equipment room on one floor. Anyway from what you have said you currently have the need for two subnets.

Your first consideration should be future flexibility. To maximize this you need to determine how many addresses (hosts) are needed in each subnet. Make your subnets fit the largest number of machines in the largest segment (make sure you look at near future needs). For example if you need 26 hosts use a /27 not a /26 Next is do you require a dynamic routing protocol (in you example it would seem you do not - only one router)? Assuming you need less then 62 hosts in each subnet and have no need to use dynamic routing (RIP, OSPF or ISIS) you should subnet with /26. Don't assign the first and second subnet (eg .0/26 and .64/26). Use the fist and third subnets (eg .0/26 and .128/26) This way you can move the netmask if required.

Next you need to determine a host number policy (something simple)
- host start at the bottom
- servers start from the end
- always avoid logical network and broadcast addresses

I have assumed that you have two ethernet ports on the router (or you would have less reason to subnet at all)

---------------------| .0/26

    .1/26 |
  R--------------> to ISP
.129/26 |

--------------------| .128/26

If you would like further assistance e-mail me directly

Hope this is a helps, with the info provided this is all the help I can be