Datagrams cannot always be forwarded to their destinations. Hardware failures, improper protocol configuration, down interfaces and incorrect routing information are some of the reasons successful delivery may not be possible. In these cases, ICMP delivers back to the sender a destination unreachable message indicating to the sender that the datagram could not be properly forwarded.

Figure shows an ICMP destination unreachable message header. The value of 3 in the type field indicates it is a destination unreachable message. The code value indicates the reason the packet could not be delivered. Figure has a code value of 0, indicating the network was unreachable. Figure shows the meaning for each possible code value in a destination unreachable message.

A destination unreachable message may also be sent when packet fragmentation is required in order to forward a packet. Fragmentation is usually necessary when a datagram is forwarded from a Token-Ring network to an Ethernet network. If the datagram does not allow fragmentation, the packet cannot be forwarded, so a destination unreachable message will be sent. Destination unreachable messages may also be generated if IP related services such as FTP or Web services are unavailable. To effectively troubleshoot an IP network, it is necessary to understand the various causes of ICMP destination unreachable messages.