Internet Core Protocols: The Definitive Guide

Destination Unreachable error messages

A Destination Unreachable error message can signify any number of problems. It can mean that a router was unable to find a path to a remote system, or it can mean that a port number on the destination system is currently unavailable, or it can signal a variety of other problems.

In order to provide more-detailed reporting, the Destination Unreachable message provides a variety of submessages (using the ICMP Message Code field as described later in Section 5.2.1.1). The major Destination Unreachable submessages documented in the various ICMP RFCs include:

Network Unreachable

This error message means that no route for the destination network could be found in the routing table on the reporting router, and is commonly seen when a user tries to connect to a private address that is non-routable across the Internet. This message can also result when datagrams are sent to a router that has corrupt or out-of-date routing tables.

For example, Figure 5.1 shows Ferret (192.168.10.10) trying to send an IP datagram to 192.168.30.10. Since the default route on Ferret points to Sasquatch, Ferret sends the datagram to that router for forwarding. However, Sasquatch does not have a routing table entry for the 192.168.30.0 network, so it returns an ICMP Destination Unreachable: Network Unreachable error message back to the sender.

Figure 5.1. A typical scenario for ...