ping
The ping utility is a command-line diagnostic tool used to test whether a host can communicate with another host on the same network or on a remote network. It is the basic tool to use to establish that a link is working.
netstat
Used to investigate open ports and connections on the local host. You can use this tool to verify whether file sharing or email ports are open on a server and whether other clients are connecting to them.
tracert
To trace the route a packet of information takes to get to its target
nslookup
Can help you troubleshoot DNS if you suspect a problem with name resolution