PING

About PING

Ping is an application to identify the status of host on a network (Ping sweep is sending ICMP Echo (Requests) to a large block of addresses to quickly find the "LIVE" host on a network). Ping uses network layer to send packets on network to check host is alive or not, to do this, it sends TTL packets (time to live) and after sending packets on network, it starts a counter to wait for response from host for a specific time (it sets to the worst case) means in that time period host should be responding to their packets otherwise these packets are lost in a network and admin assumes that host is dead.

ICMP

ICMP	Message Type	Identifier Description Supported?
Echo Reply	0	The ping reply packet. Sent in response to Echo Requests.
Destination Unreachable	3	Sent by intermediate devices (routers, and so on) when target address is unavailable. Subcodes include 0—Network Unreachable 1—Host Unreachable 3—Port Unreachable 5—Source route failure 6—Destination network unknown 7—Destination host unknown 9—Network Admin Prohibited 10—Host Admin Prohibited 13—Admin Prohibited
Redirect	5	A packet tells what the route packet should use.
Time Exceeded	11	A packet failed to reached their destination.