Which ICMP message type code indicates fragmentation needed but DF bit set?
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A. B. C. D.C.
The correct answer to the question is option C, Type 3, Code 4.
ICMP (Internet Control Message Protocol) is a network protocol used to send error messages and operational information about network conditions. It is typically used by network devices such as routers and firewalls to communicate with each other.
When an IP packet is sent, it can be fragmented into smaller packets if the size of the packet exceeds the Maximum Transmission Unit (MTU) of the network path. The DF (Don't Fragment) bit in the IP header is used to indicate whether or not a packet can be fragmented. If the DF bit is set to 1, it means that the packet cannot be fragmented.
If a packet needs to be fragmented but the DF bit is set to 1, the sender will receive an ICMP error message indicating that fragmentation is needed but the DF bit is set. This message is called "Destination Unreachable - Fragmentation Needed and Don't Fragment Bit Set", and its ICMP type and code are 3 and 4, respectively.
Let's take a closer look at the other options:
A. Type 3, Code 0 - This ICMP message is used to indicate that the destination network is unreachable.
B. Type 4, Code 2 - This ICMP message is used to indicate that the sender has received an ICMP message that it cannot interpret.
D. Type 8, Code 0 - This ICMP message is known as an Echo Request and is used for network testing and troubleshooting. It is often referred to as a "ping" packet.
In conclusion, option C, Type 3, Code 4, is the correct answer because it indicates that fragmentation is needed but the DF bit is set.