Hey @lcw Thank you for your reply.
I got a direct link between two servers as our admin said.  So I am running only with two servers.
@mrshenli suggested running DDP to check. I tried DDP example 8 and it works perfectly.
But again I got “host unreachable” error for the parameter server example.
I will try your suggestion as well. But since DDP runs correctly, maybe it would be a clue for you. I tried it by gloo and NCCL backend both work.