Netstat is used to check which ports are open on the local system. There is also a tool called nmap which scans a remote system for open ports. For example
nmap hostaddress -p0-65535
checks all the possible ports on the specified address.
Tcpdump is a tool that sniffs data from the network traffic. To record data from a specific host type:
tcpdump dst host <hostaddress>
You can also filter out specific protocols like http:
tcpdump tcp port http.