In order for your first question:

Computers usually use a method called HOST DETECTION to validate a host's existence. If the ICMP gets closed on a network (for example), then many of the Scanners and Tracers gets failed.

so in this situation you can use other Advanced Host Detection (and verification) to perform host detection, for example:

Sending a SYN packet to a closed port: In this situation the target host will discard the sent packet and you can detect its existence by this activity.

If you need more details, I can continue ...

Such techniques usually used on the Network Scanners and you can develop them (if they're Open Source) and know programmming.