1. check your gateway is availbe
a. find the gateway ip
route
b. Check whether gateway is availble.
ping gatewayIP
2. Check your DNS service is available
nslook baidu.com
It will resolve the url name or host machine name to ip if the name server works.
3. Check check whether the server can be routed
routetrace serviceip
4. Check the service portal
nmap servername
nmap -sTRU -P 1-1000,12345 baidu.com
http://www.linuxjournal.com/article/456