How do I troubleshoot a scan that reports "No alive hosts found"?

This article describes how to use the diagnostics tool on a Scanner Appliance to check network-layer reachability when a scan finishes with "No alive hosts found," so you can determine whether the issue sits in the scan profile configuration or in the network path.

A scan that finishes with "No alive hosts found" does not necessarily mean the targets are down. It usually means the Scanner Appliance could not confirm any hosts were reachable using the discovery methods configured in the scan profile. Before adjusting scan profile settings, it is worth confirming the fundamentals: Can the Scanner Appliance actually reach the scanned network at the network layer?

The diagnostics tool on the Scanner Appliance lets you send an on-demand ping from the appliance itself to one or more target IP addresses. This isolates the problem to one of two causes:

• If the Scanner Appliance can reach the target, the issue is more likely scan profile configuration, like the discovery method, the ports used, or the ICMP vs. TCP SYN/SYN-ACK settings.
• If the Scanner Appliance cannot reach the target, the issue is network-layer, for example routing, firewall rules, VLAN or subnet segmentation, or ICMP being blocked somewhere between the Scanner Appliance and the target.

Running this check early in a ticket saves time. It prevents back-and-forth profile tuning when the real blocker is a firewall rule on the customer's side.

Before you start

• Confirm which Scanner Appliance is assigned to the scan or schedule that returned the warning.
• Have the exact target IP addresses ready, or a representative sample from the affected IP range. For large ranges, pick two or three hosts you would expect to be alive.
• Know whether the target subnet is local to the Scanner Appliance or reachable through routing or firewall hops, since that context helps interpret a failed ping.

Run a manual ping

1. Open the Scanner Appliance console for the appliance in question.
2. Click Diagnostics > Manual.
3. Enter the target IP address, or addresses, that you want to test.
4. Run the ping test and wait for it to complete.
5. Review the response for each target.
6. Repeat steps 3 to 5 for a few IP addresses across the range if the scan covered a broad network, rather than testing only a single host. This helps distinguish a host-specific issue from a broader network path problem.

Interpret the results

For each target, a reply received means the Scanner Appliance has network-layer reachability to that host. No reply or a timeout means the Scanner Appliance cannot reach the host over ICMP from its current network position.