Netstat command is helpful for display information related to network connections, both incoming and outgoing , routing tables, interface statistics security issues connections and so on.

Exim is a message transfer agent (MTA) for hosts that are highly comfortable and running in Unix or Linux Operating system.

bind-utils package is very helpful and its a collection of client side program includes the packages are nslookup, host and dig.

Uptime command is very simple and useful in Linux , Give information about system current time, how long the system has been running,