How to manage unknown workstation

How to manage unknown workstation

Hi,
I scan entire network 192.168.1.x and find all switches in this range. All my switches in this range are 30 devices. But I have some IP that should pingable from SDP server. Now when I ping an IP that put in the unknown workstation, I received this response:

Pinging 192.168.1.21 with 32 bytes of data:
Reply from 172.20.3.4: Destination host unreachable.
Reply from 172.20.3.4: Destination host unreachable.
Reply from 172.20.3.4: Destination host unreachable.
Reply from 172.20.3.4: Destination host unreachable. 

  I think SDP can't detect between Ping Ok with  Destination host unreachable. I think it's a bug. Please solve it.

Thanks
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