The gateway usually has a "default route" entry, this entry is used when the gateway doesn't know where the network is. At that point, the gateway should be able to figure out where to forward the ICMP Echo request. When you open a DOS command prompt and type "ping 200.200.200.200", assuming that your workstation is NOT part of that network, then it would forward the ICMP Echo request to the gateway that's configured in your TCP/IP properties. Please read on as the following example will help you understand all the above. Let's look at the packet structure of an ICMP destination unreachable packet: You can check to see if all routers and gateways are configured properly and have their routing tables updated and synchronised. These ICMP messages are most useful when trying to troubleshoot a network. In another case, when a packet received must be fragmented to be forwarded by a gateway but the "Don't Fragment" flag (DF) is on, the gateway must discard the packet and send an ICMP destination fragmentation needed and DF set unreachable message to the source host. If, in the destination host, the IP module cannot deliver the packet because the indicated protocol module or process port is not active, the destination host may send an ICMP destination protocol / port unreachable message to the source host.
The ICMP - Destination host unreachable message is one which a user would usually get from the remote gateway when the destination host is unreachable.
#Destination host unreachable how to#
The ICMP - Destination net unreachable message is one which a user would usually get from the gateway when it doesn't know how to get to a particular network. It goes something like this: ICMP Destination unreachable.
#Destination host unreachable code#
To make sure you don't get confused, keep one thing in mind: The ICMP Destination unreachable is a generic ICMP message, the different code values or messages which are part of it are there to clarify the type of "Destination unreachable" message was received. The table below shows an brief summary of the available messages and their code value contain in the ICMP header:
This article will analyse all six destination unreachable messages and explain which occasions each message is used. The ' ICMP Destination unreachable' message is quite interesting, because it doesn't actually contain one message, but infact six! This means that the ICMP Destination unreachable futher breaks down into 6 different messages.
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