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Networking Fundamentals · Intermediate

ICMPv6

The IPv6 version of ICMP (next header 58), which handles errors, diagnostics, Neighbor Discovery and multicast listener discovery.

Definition

Beyond echo and error messages, ICMPv6 carries NDP and MLD, so IPv6 cannot function if ICMPv6 is blocked. RFC 4890 lists which ICMPv6 types firewalls should permit, including Packet Too Big (2), which is essential for PMTUD.

Example

ICMPv6 type 128 is Echo Request and 129 is Echo Reply.

Exam tip

Never block all ICMPv6; Neighbor Discovery and Packet Too Big depend on it.

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