Basic: ICMP Types and Codes

You know there are more to the humble PING than meets the eye. Many of us never think of it more than trying to check whether we have connectivity to the Internet or switch or other systems on the LAN.

When we PING any hostname or IP or domain name, we are actually using ICMP (Internet Control Message Protocol) to send an echo request packet to the target host, gateway or router and if the target is alive or has permission to respond, it will send back an echo reply.

Below is a figure of the ICMP Packet which is sent and recieved. You will observe that it has fields for ICMP Type and Code.

If you use any packet capture tools to study the packets being sent, the Type and Code fields says a lot about the type of communication and status.

Type	Code	Description
0 – Echo Reply	0	Echo reply (used to ping)
1 and 2		Reserved
3 – Destination Unreachable	0	Destination network unreachable
	1	Destination host unreachable
	2	Destination protocol unreachable
	3	Destination port unreachable
	4	Fragmentation required, and DF flag set
	5	Source route failed
	6	Destination network unknown
	7	Destination host unknown
	8	Source host isolated
	9	Network administratively prohibited
	10	Host administratively prohibited
	11	Network unreachable for TOS
	12	Host unreachable for TOS
	13	Communication administratively prohibited
	14	Host Precedence Violation
	15	Precedence cutoff in effect
4 – Source Quench	0	Source quench (congestion control)
5 – Redirect Message	0	Redirect Datagram for the Network
	1	Redirect Datagram for the Host
	2	Redirect Datagram for the TOS & network
	3	Redirect Datagram for the TOS & host
6		Alternate Host Address
7		Reserved
8 – Echo Request	0	Echo request (used to ping)
9 – Router Advertisement	0	Router Advertisement
10 – Router Solicitation	0	Router discovery/selection/solicitation
11 – Time Exceeded[6]	0	TTL expired in transit
	1	Fragment reassembly time exceeded
12 – Parameter Problem: Bad IP header	0	Pointer indicates the error
	1	Missing a required option
	2	Bad length
13 – Timestamp	0	Timestamp
14 – Timestamp Reply	0	Timestamp reply
15 – Information Request	0	Information Request
16 – Information Reply	0	Information Reply
17 – Address Mask Request	0	Address Mask Request
18 – Address Mask Reply	0	Address Mask Reply
19		Reserved for security
20 through 29		Reserved for robustness experiment
30 – Traceroute	0	Information Request
31		Datagram Conversion Error
32		Mobile Host Redirect
33		Where-Are-You (originally meant for IPv6)
34		Here-I-Am (originally meant for IPv6)
35		Mobile Registration Request
36		Mobile Registration Reply
37		Domain Name Request
38		Domain Name Reply
39		SKIP Algorithm Discovery Protocol, Simple Key-Management for Internet Protocol
40		Photuris, Security failures
41		ICMP for experimental mobility protocols such as Seamoby [RFC4065]
42 through 255		Reserved