Network engineers design and build computer networks. Employers are looking for strongly skilled individuals to design their networks on a high level, leaving support and maintenance to network administrators. During the interview process be prepared for many technical questions encompassing all of network theory.
How would you know if a port is open or not.
Try to ping the port. The answer they were looking for was probably a port sniffer. Or network monitoring tool like wireshark.
On the server itself, use "netstat -an" to check to see which ports are listening From outside, just "telnet host port" to see if the connection is refused, accepted, or timeouts
Using nmap is the best way I've found with the most success rate overall. Nmap can do TCP and UDP based port scans.