Ping

ping is a tool written by the late Mike Muus to measure latency and round-trip time between two hosts connected by an Internet, often it is also used to simply debug if the other host is up. It works by sending an ICMP type 8 packet to the remote host which replies with an ICMP type 0 packet. Sometimes there is firewalls preventing a ping from receiving a reply.

This would look like so:

$ ping -c 1 www.yashy.com
PING mail.yashy.com (206.248.137.44): 56 data bytes
--- mail.yashy.com ping statistics ---
1 packets transmitted, 0 packets received, 100.0% packet loss

One of the original DoS was a simple ping flood. If you have more bandwidth then your victim, you can do

# ping -f example.com.

and slow your victim to a grinding halt.

A similar tool to ping would be traceroute.