Ping Fundamentals.

1.PC to PC connected directly  using crossover cable.
   Will Ping if IP subnet is same.
   Will not Ping if IP Subnet differs.

2.Two PC's connected to a Switch ,each in a different Subnet
   will not ping as Switch cannot do routing.Even on L3 Switch
   with IP Routing enabled Ping will not work as the Host/PC
   needs a default Gateway and the switch interface cannot be
   assigned gateway IP like a Router and int vlan 1 / native vlan
   can be assigned only one IP (wonder if vlan interfaces can
   be assigned secondary IP's). To do this on an L3 switch
   add interface to another VLAN and give that interface
   an IP and enable ip routing which would be inter VLAN
   routing on an L3 switch.

3.If an IP is assigned to a Router interface and Ping is tested
   from same Router, Ping will not work until there is a line
   detect / keepalives that is a Host/Router is connected to other
   end.This is needed even to test Ping from same Router.

4.Ping between Two switches possible.If
      > both connecting interface's belong to same VLAN
         Number on there respective switch
      > IP Subnet is same.

5.A Loopback Interface can be pinged from another Router
   as long as there is a Route to the loopback IP subnet in
   the Routing Table of the Router from which the Ping is
   issued.The Route can be got from a Routing protocol
   from the Router having the loopback int or a static or
   default route has to be added.
   Simply No Route in the Routing Table no Path hence no Ping.

6.Two Routers connected to a switch with IP in same subnet will
   ping each other.Connected directly will also ping as along gas
   subnet is same.

7.  Ping to a Router/Host  will not work if there is not a reverse 
    path.This is the most fundamental routing fact.Packets need 
    to know a route back to the originating ping or any other form 
    of communication.Hosts/PC's will go back to the gateway which 
    will be a Router and the Router must have a route back in its 
    routing table.The route can be from a routing protocol or 
    default or static route.