What does 'failed to resolve IP address' mean?

The first thing that happens when using any of the tools is that the entered hostname (e.g. google.com) is resolved to the corresponding IP address. The IP address resolution is performed using the configured DNS server. When it fails, you get the "failed to resolve IP address" message.

Typically this happens when:

1. You lose the Internet connection so the DNS server cannot be queried for the IP address.
2. You mistype the domain name and type something that doesn't exist - e.g. "google.comm" for which the IP cannot be resolved. Note that you also get this error when you mistype the IP address, e.g. 192.168.01 (missing dot) because the app doesn't recognise the IP and tries to resolve it as a domain name.