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I have networks in 2 separate buildings. Each has its own private IP range. We're installing a private fiber link between the two buildings. Do I just need to adjust the subnet mask so they can see devices in each building or do I need to re-ip one side to match the other? Currently it's:

Building A: 10.0.11.x 255.255.255.0 Building B: 10.0.12.x 255.255.255.0

Would changing the subnet mask to 255.255.0.0 be correct or is there a better option?

Thanks in advance for any help!

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    Routers route between networks. Extending layer-2 between the building will let layer-2 problems affect both buildings. Simply put in a router to connect both networks.
    – Ron Maupin
    Commented Sep 7, 2023 at 15:48
  • Please edit your question to include more details, such as your equipment types, the distance between the buildings, etc. There are different approaches that depend on these variables.
    – Ron Trunk
    Commented Sep 7, 2023 at 15:53
  • Has any answer solved your question? Then please accept it or your question will keep popping up here forever. Please also consider voting for useful answers.
    – Zac67
    Commented Oct 8, 2023 at 9:36

1 Answer 1

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Do I just need to adjust the subnet mask so they can see devices in each building or do I need to re-ip one side to match the other?

Either works. Your options:

  1. change the subnet mask to /21 (255.255.248,0) or shorter on all connected hosts/nodes and simply connect the switches
  2. (recommended) use a router or L3 switch to route between /24 subnets

Option 1 wastes quite a few /24 subnets in between but is the easier option.

Since the link leaves and re-enters your premises you might consider using the routed option 2 plus intermediate encryption (VPN or MACsec). Fiber is not very easy to tap, but it isn't that hard either, especially if you don't implement tight monitoring on the link.

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    With option 1 you need to change the subnetmask on ALL hosts in building 1 and 2.
    – MatsK
    Commented Oct 8, 2023 at 15:25

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