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Networking Fundamentals · Intermediate

Destination Unreachable

ICMP type 3 message indicating a packet could not be delivered, with codes giving the reason.

Definition

Common codes are 0 network unreachable, 1 host unreachable, 3 port unreachable, 4 fragmentation needed and DF set, and 13 administratively prohibited (filtered). Port unreachable is generated by the destination host for closed UDP ports.

Example

A traceroute on Linux ends when the destination returns ICMP port unreachable to its UDP probe.

Exam tip

Code 13 points to an ACL or firewall; code 4 points to an MTU problem.

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