Protocols

Protocols - currenty most protocols that start with I operate at level 3 (Except for IMAP)- ICMP, IGRP, IGMP, IPSec

ICMP - Internet Control Messaging Protocol
Responsible for echoing utillities like Ping, Tracert, traceroute
Ping is used to test basic physical connectivity for local and remote hosts
If a ping fails perhaps use Tracert / Traceroute to see the hops to diagnose the problem
where the ping fails
Tracert for Windows, Traceroute for Linux - Chases hops through routers
Ping a local computer on my network - If I can reach it, great!
Then I'll ping a remorte computer - One on the other side of the router / If I can't raeach that remote host I'll use Tracert/Traceroute so I can see the message leave from me
Goes to Router 1, Goes to Router 2, Then It doesn't look like it will pass router 3