ICMP

Type - These 8 bits determine the ICMP message type a shown in the Table 1.

Code - These 8 bits determine the ICMP message code as shown in the Table 1.

Checksum - These 16-bits are the ones's complement of the one's complement sum of the ICMP message starting with the ICMP Type

Type	Code	Description	Query	Error
0	0	Echo reply (Ping reply)	*
3	0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15	Destination unreachable Network unreachable Host unreachable Protocol unreachable Port unreachable Fragmentation needed but don’t fragment bit set Source route failed Destination network unknown Destination host unknown Source host isolated (obsolete) Destination network administratively prohibited Destination host administratively prohibited Network unreachable for TOS Host unreachable for TOS Communication adminstratively prohibited by filtering Host pecedence violation Precedence cutoff in effect		* * * * * * * * * * * * * * * *
4	0	Source quench (elementary flow control)		*
5	0 1 2 3	Redirect Redirect for network Redirect for host Redirect for type-of-service and network Redirect for type-of-service and host		* * * *
8	0	Echo request (Ping request)	*
9 10	0 0	Router advertisement Router solicitation	* *
11	0 1	Time exceeded: Time-to-live equals 0 during transmit Time-to-live equals 0 during re-assembly		* *
12	0 1	Parameter problem: IP header bad (catchall error) Required option missing		* *
13 14	0 0	Timestamp request Timestamp reply	* *
15 16	0 0	Information request Information reply	* *
17 18	0 0	Address mask request Address mask reply	* *

It can be seen from this table that Ping uses an ICMP message:
type 0, code 0 for a reply and
type 8, code 0 (for a request)

The full specifications for the ICMP messages (rfc792)
can be viewed from: http://www.cis.ohio-state.edu/htbin/rfc/rfc792.html

type (8-bit)	code (8-bit)	checksum (16-bit)
contents depends on type and code

<--- IP datagram --->
IP header	ICMP header