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
Can External scanner be used for authenticated scans?
Yes. But you should review if that's what you're after. Usually, auth ports are not exposed to the outside.
See more here: Scanning Strategies and Best Practices
What if I scan my web application IP using external scanner?
Can you provide details, the question isn't very clear.
I meant to ask what if I put my web app IP in Qualys VM module and start vulnerability scan on it? Will its results be same as using Qualys web application scanner(Qualys WAS module) on web app? Because at least to me, logically it looks same to me.
WAS scans are quite different from VM scan.s A web application scan will inspect the web application itself (and also APIs) to identify application-level vulnerabilities like XSS, etc. A VM scan will scan the device to identify device-level (e.g. OS, database, network) issues like those reported on Patch Tuesday, etc. See Web Application Scanning | Qualys, Inc. and Vulnerability Management | Qualys, Inc.
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