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I have a single ethernet cable between two buildings. If want to run two subnets, 192.158.1.x and 172.20.1.x. The two devices on the two separate subnets are connected to a layer 2 switch at the far end. Then I have two routers, one on each subnet, at the near end. Will there be any issues? I know its not ideal but to put a second cable means a great deal of excavation work to dig out the single cable and replace with two. I have tried an ethernet splitter to use the unused pairs but could not get it to work.

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    "to put a second cable means a great deal of excavation work to dig out the single cable and replace with two." If the cable installer did it correctly in the first place, there is a pull string to pull an additional cable and another pull string. If the cable installer did it incorrectly with no pull string, then you can use the first cable as a pull string to pull two or more cables and a proper pull string.
    – Ron Maupin
    Commented Jun 18, 2023 at 18:49
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    I guess you've never seen direct burial cable. (read: they didn't use conduit)
    – Ricky
    Commented Jun 18, 2023 at 20:19

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You can actually run any number of IP subnets on a single switch or link, but that rarely makes real sense.

Usually you'd want the subnets to be really separated which requires VLANs and tagging. Without, any host can join any subnet by simple configuration. Also, you can't use DHCP (easily) when you just connect subnets together.

I have tried an ethernet splitter to use the unused pairs but could not get it to work.

Splitting a four-pair cable into two-pair sets only works for 10 or 100 Mbit/s. Gigabit requires all four pairs to work.

As long as bandwidth is not a concern, VLANs will work nicely. Without VLANs, the above caveats apply.

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  • I agree that the proper method to use would be to either use the existing cable as a pull string to pull more cables (preferably fiber optic) and/or to use VLANs to transport separate networks over the single cable in use. Commented Jun 18, 2023 at 19:34

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