ping command

[tbaher]

there are many reasons why you cant get a ping reply on stuff
if it says unreachable, then you are probably behind a firewall/proxy.
What this means is that your browser tells another computer to get so and so for it which then gets sent to u. (hence the word proxy .. middleman)
this is how u can browse but not get a "path" to the other host, i.e. it is not reachable
if the request simply times out then it means that the server is "reachable" but doesnt "pong" when you "ping" in a set amount of time. This amount of time is called timeout.
There are many reasons:
-It takes too long to reply so it times out. (it will usually reply to 1 or more of the 4 requests though)
-It doesnt have a ping server or is blocking the port. That basically means that it "refuses" to reply to the ping and just ignores it (or just isnt listening for pings)
-Something else is wrong. Like maybe the whole computer isnt up and running properly.

In your particular case, that whole unreachable thing means that you are not on the same network, you are not using NAT, you are probably using a firewall/proxy even if you are unaware of it (there are invisible firewalls/proxies by your ISP maybe) and that there is no DIRECT PATH to "www.yahoo.com" and thus to the internet.
Nothing to worry about unless some program doesnt work (like some online game)

[lizadonald]

tbaher, i read your answer again and again, i understood all except one line i.e. "you are not using NAT". What do u mean by "NAT"

[tbaher]

being behind a router

http://www.webopedia.com/TERM/N/NAT.html

[lizadonald]

tbaher, why our post has been removed from chit-chat to general gaming sub forum, are we playing any sort of game.