Today’s we are discuss ICMP unreachable codes.
ICMP Echo Request and Echo Reply
The ICMP Echo Request and Echo Reply messages are sent and received by the ping command. In fact, when people say they sent a ping packet, they really mean that they sent an ICMP Echo Request. These two messages are self-explanatory. The Echo Request simply means that the host to which it is addressed should reply to the packet. The Echo Reply is the ICMP message type that should be used in the reply. The Echo Request includes some data that can be specified by the ping command, whatever data is sent in the Echo Request is sent back in the Echo Reply.
ICMP Unreachable Codes
|Unreachable Code||When It is Used||What It Typically Is Sent By|
There is no match in a routing table for the packet’s destination.
The packet can be routed to a router connected to the destination subnet, but the host is not responding.
The packet has the Don’t Fragment bit set, and a router must fragment to forward the packet.
The packet is delivered to the destination host, but the transport layer protocol is not available on that host.
The packet is delivered to the destination host, but the destination port has not been opened by an application.