Which statement about ICMP is true?

• A. It is not used for reachability checks.
• B. It is never trusted by networks.
• C. It is only used on IPv6.
• D. It is often trusted and allowed through firewalls.

Answer: (D)

ICMP exists to provide feedback about the delivery of IP packets, including reachability and error information. Because networks rely on these signals for diagnostics and for functions like path MTU discovery, many operators configure firewalls to allow some ICMP traffic to pass. This is why the statement that ICMP is often trusted and allowed through firewalls is accurate: you typically see rules that permit essential ICMP types (like Echo Request/Reply and certain error messages) while still using rate limiting or type-based filtering to reduce abuse.

Understanding the other options helps solidify this: ICMP is indeed used for reachability checks (ping is the classic example), so saying it’s not used for reachability checks is false. ICMP is not IPv6-only; it exists in IPv4 as well (with ICMPv4) and remains a vital part of IPv6 (ICMPv6) too, so claiming it’s exclusive to IPv6 is incorrect. While some networks may block ICMP for security, in practice it’s often tolerated or selectively allowed to preserve troubleshooting capabilities and proper network operation.