ICMP Destination Unreachable
This value is the sum of all identified events in the ICMP Events display.
Possible codes:
0=net unreachable
1=host unreachable
2=protocol unreachable
3=port unreachable
4=fragmentation needed and DF set
5=source route failed
If, according to the information in a router or gateway's routing tables, the network specified in the Internet destination field of a datagram is unreachable, e.g., the distance to the network is infinity, the router or gateway may send a destination unreachable message to the Internet source host of the datagram. In addition, in some networks, the router or gateway may be able to determine if the Internet destination host is unreachable. Routers\Gateways in these networks may send destination unreachable messages to the source host when the destination host is unreachable.
If, in the destination host, the IP module cannot deliver the datagram because the indicated protocol module or process port is not active, the destination host may send a destination unreachable message to the source host.
Another case is when a datagram must be fragmented to be forwarded by a gateway yet the Don't Fragment flag is on. In this case the gateway must discard the datagram and may return a destination unreachable message.
Codes 0, 1, 4, and 5 may be received from a router or gateway. Codes 2 and 3 may be received from a host.
Possible reasons for the event:
The router's routing tables are misconfigured.
The destination just does not exist.
The destination has been excluded as a possible valid address (i.e. disallowed).
The service requested is not available on the destination system.