The TCP/IP Guide - ICMPv4 Destination Unreachable Messages

There are many different reasons why it may not be possible for a datagram to reach its destination. Some of these may be due to erroneous parameters (like the invalid IP address example mentioned above.) A router might have a problem reaching a particular network for whatever reason. There can also be other more “esoteric” reasons as well why a datagram cannot be delivered.

For this reason, the ICMPv4 Destination Unreachable message type can really be considered a class of related error messages. The receipt of a Destination Unreachable message tells a device that the datagram it sent couldn't be delivered, and the reason for the non-delivery is indicated by the Code field in the ICMP header. Table 89 shows the different Code values, corresponding message subtypes and a brief explanation of each.

**Table 89: ICMPv4 *Destination Unreachable* Message Subtypes**
Code Value	*Message Subtype*	Description
0	*Network Unreachable*	The datagram could not be delivered to the network specified in the network ID portion of the IP address. Usually means a problem with routing but could also be caused by a bad address.
1	*Host Unreachable*	The datagram was delivered to the network specified in the network ID portion of the IP address but could not be sent to the specific host indicated in the address. Again, this usually implies a routing issue.
2	*Protocol Unreachable*	The protocol specified in the Protocol field was invalid for the host to which the datagram was delivered.
3	*Port Unreachable*	The destination port specified in the UDP or TCP header was invalid.
4	*Fragmentation Needed and DF Set*	This is one of those “esoteric” codes. J Normally, an IPv4 router will automatically fragment a datagram that it receives if it is too large for the maximum transmission unit (MTU) of the next physical network link the datagram needs to traverse. However, if the DF (Don't Fragment) flag is set in the IP header, this means the sender of the datagram does not *want* the datagram ever to be fragmented. This puts the router between the proverbial rock and hard place, and it will be forced to drop the datagram and send an error message with this code. This message type is most often used in a “clever” way, by intentionally sending messages of increasing size to discover the maximum transmission size that a link can handle. This process is called MTU path discovery.
5	*Source Route Failed*	Generated if a source route was specified for the datagram in an option but a router could not forward the datagram to the next step in the route.
6	*Destination Network Unknown*	Not used; Code 0 is used instead.
7	*Destination Host Unknown*	The host specified is not known. This is usually generated by a router local to the destination host and usually means a bad address.
8	*Source Host Isolated*	Obsolete, no longer used.
9	*Communication with Destination Network is Administratively Prohibited*	The source device is not allowed to send to the network where the destination device is located.
10	*Communication with Destination Host is Administratively Prohibited*	The source device is allowed to send to the network where the destination device is located, but not that particular device.
11	*Destination Network Unreachable for Type of Service*	The network specified in the IP address cannot be reached due to inability to provide service specified in the Type Of Service field of the datagram header.
12	*Destination Host Unreachable for Type of Service*	The destination host specified in the IP address cannot be reached due to inability to provide service specified in the datagram's Type Of Service field.
13	*Communication Administratively Prohibited*	The datagram could not be forwarded due to filtering that blocks the message based on its contents.
14	*Host Precedence Violation*	Sent by a first-hop router (the first router to handle a sent datagram) when the Precedence value in the Type Of Service field is not permitted.
15	*Precedence Cutoff In Effect*	Sent by a router when receiving a datagram whose Precedence value (priority) is lower than the minimum allowed for the network at that time.

As you can see in that table, not all of these codes are actively used at this time. For example, code 8 is obsolete and code 0 is used instead of 6. Also, some of the higher numbers related to the Type Of Service field aren't actively used because Type Of Service isn't actively used.

Please Whitelist This Site?