|If you're trying to ping something on the internet, it could be it's set to not reply by whoever administers it. If you're trying to ping your friend's external IP address, his ISP could be blocking ICMP.|
If you're talking pinging across subnets within a LAN, unless there are static routes between subnets, you won't get replies since they're on a separate subnet (network).
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.