What is the possibilities what external scan can do, it is the same like internal scan?
Can I get the information what ports are open on scanned IPs?
External and internal scans are different in that they are sourced from different segments of your network.
External scans are initiated from the Internet using Qualys external scanners while an internal scan is initiated using a scanner appliance hosted inside your office. As a result they have different perspectives and may (or most likely) produce different output. Both scans will show you the list of open ports detected.
This video will help you understand scanner placements: Scanner Appliance Location
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