For security purposes, it is not possible to use the ping utility to test the reachability of our mining servers. When you try to ping Slush Pool’s mining servers, you will probably get a 100% packet loss.
Slush Pool’s mining servers are intentionally configured in this way, so an unsuccessful ping test does not mean that the servers are down or unavailable. In order to test the reachability of these hosts, please use Telnet instead.
Example of a successfully established Telnet connection:
telnet stratum.slushpool.com 3333 Trying 18.104.22.168... Connected to stratum.slushpool.com.