# Like ‹ping›, ‹traceroute› sends out ICMP echo request packets.
# However, unlike ‹ping›, it also sets the TTL field in the IP
# header -- short for Time To Live. This number is decremented by
# each router along the way, and the packet is thrown away when its
# TTL drops to 0. The router doing the dropping also helpfully sends
# back an ICMP ‘time to live exceeded’ packet.

# Someone clever noticed that you can send ICMP echo request with
# TTL set to 1 to learn the address of the first router along the
# way to the destination, then set TTL to 2 to learn about the
# second closest one, and so on. This is what ‹traceroute› does, and
# it prints information about each of the hosts it encounters. Let's
# try it on the trio of computers we pinged earlier:

# By default, ‹traceroute› tries to find a hostname for each IP
# address. But we don't know enough about DNS to worry about that
# yet. So we just ask it to print numeric addresses, using ‹-n›.

traceroute -n 1.1.1.1
traceroute -n 8.8.8.8
traceroute -n 4.2.2.2

# Now that we tried both ‹ping› and ‹traceroute›, let's try some DNS
# queries.  See ‹host.sh›.