Some possible causes for this message and recommended troubleshooting steps are described below.
The server hosting the web application must respond to vulnerability checks conducted by the scanning engine. Some causes for the server to be not responsive are:
The scanner appliance does not have network access to the web application.
A network access control rule (from a firewall, proxy server, router, etc.) doesn’t allow access to the web application. Access control rules might enable ICMP (ping) to the server, but block HTTP traffic.
The web server is down.
The IP address or FQDN has changed since the web application in your account was created or last edited.
That the IP address or FQDN was mistyped for the web application in your account.
The scanning engine could not access the port where the web application is running.
Though rare, it is entirely possible that a problem can occur and for that kind of issue our engineering team receives a notification email automatically to help us to fix the issue. To identify whether a problem may have occurred is to just look at how much time the scan lasted before you your scan returned the "No Host Alive" status. As a rule of thumb if the scan duration was more than 5 minutes then there is a high chance that an internal problem occurred. Please contact Support if you've encountered a problem.
Verify in a browser that the IP address or FQDN and the provided port loads the web application.
Verify that the web application, as defined in your account, has the correct IP/FQDN and port.
Check if the web server is connected to the network.
Check to be sure the web server is up and running.
Check with the network administrator that the web application has the necessary access (IP address and port) that allows the service access to your target IP addresses. See Check Scanner IP Addresses.