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ICMP Validity Check: Which Statements Could Be Valid?

ICMP Validity Check: Which Statements Could Be Valid?

Verified Sources
Nov 3, 2026

Internet Control Message Protocol (ICMP) is used by routers/hosts to report certain network-layer problems and provide diagnostics. Key ICMP message families include error reporting (e.g., Destination Unreachable) and routing updates/optimization via Redirect.2
To evaluate the four statements, we map each claim to what ICMP specifies (and what it does not).

Answer (valid statements): (ii) and (iv).

  • (ii) is valid because ICMP Redirect is sent when a router believes the sender should use a better next hop for subsequent packets.2
  • (iv) is valid because “Destination Unreachable” reports that the destination (host/network/port) can’t be reached.2
  • (i) is not valid: ICMP is error reporting, but it does not “report all errors” that occur during transmission.
  • (iii) is not valid as written: ICMP may inform hosts about routing improvements (Redirect), but it does not broadly “inform routers when an incorrect path has been taken.” Redirect is from a router to a host.2

Footnotes

  1. ICMP Message Definitions for Isolating Network Connectivity Issues - Includes definition for “Destination Unreachable”. ↩ ↩2

  2. Understand ICMP Redirect Messages - Cisco - Explains redirect behavior and conditions, referencing RFC 792. ↩ ↩2 ↩3

  3. ICMP Redirect Attack Lab - SEED Labs (PDF) - States redirects inform the sender about a different router for subsequent packets and are sent by routers to the sender. ↩ ↩2

  4. Internet Control Message Protocol (ICMP) - LiveAction - Describes Destination Unreachable and its troubleshooting meaning. ↩

  5. What is ICMP (Internet Control Message Protocol)? - ClouDNS Blog - Discusses ICMP limitations and that it is used for specific error reporting/diagnostics (not all errors). ↩

ICMP Message Types Explained (Destination Unreachable, Time Exceeded, Redirect, etc.)

Concepts to use for the multiple statements

You can classify each statement by whether it matches one of these ICMP roles:

  • ICMP Redirect: sent when a router believes the host is using a suboptimal first hop and should send future packets to a different router.2
  • Destination Unreachable: indicates the destination host/network/port cannot be reached; codes refine why (e.g., net unreachable, host unreachable, port unreachable).2
  • ICMP error reporting scope: ICMP is designed for specific network-layer issues; it is not a universal “all transmission errors” reporter.

Footnotes

  1. Understand ICMP Redirect Messages - Cisco - Explains redirect behavior and conditions, referencing RFC 792. ↩

  2. ICMP Redirect Attack Lab - SEED Labs (PDF) - States redirects inform the sender about a different router for subsequent packets and are sent by routers to the sender. ↩

  3. ICMP Message Definitions for Isolating Network Connectivity Issues - Includes definition for “Destination Unreachable”. ↩

  4. Internet Control Message Protocol (ICMP) - LiveAction - Describes Destination Unreachable and its troubleshooting meaning. ↩

  5. What is ICMP (Internet Control Message Protocol)? - ClouDNS Blog - Discusses ICMP limitations and that it is used for specific error reporting/diagnostics (not all errors). ↩

Decide which statements are valid (logic workflow)

  1. 1
    Step 1

    Map each statement to a known ICMP type/category: Redirect, Destination Unreachable, or generic error reporting.

  2. 2
    Step 2

    ICMP Redirect is sent by a router to a host to update routing for subsequent packets, not to ‘inform routers’ broadly.2

    Footnotes

    1. Understand ICMP Redirect Messages - Cisco - Explains redirect behavior and conditions, referencing RFC 792. ↩

    2. ICMP Redirect Attack Lab - SEED Labs (PDF) - States redirects inform the sender about a different router for subsequent packets and are sent by routers to the sender. ↩

  3. 3
    Step 3

    ‘Destination Unreachable’ specifically indicates an unreachable destination; ‘all errors during transmission’ is too broad for ICMP’s documented scope.2

    Footnotes

    1. ICMP Message Definitions for Isolating Network Connectivity Issues - Includes definition for “Destination Unreachable”. ↩

    2. What is ICMP (Internet Control Message Protocol)? - ClouDNS Blog - Discusses ICMP limitations and that it is used for specific error reporting/diagnostics (not all errors). ↩

  4. 4
    Step 4

    Choose (ii) if it matches Redirect semantics and (iv) if it matches Destination Unreachable semantics; reject the others.

Statement-by-statement evaluation

(i) “It reports all errors which occur during transmission.”

This is not valid. While ICMP provides standardized error reporting (e.g., unreachable destinations), its role is limited to certain network-layer delivery problems rather than every transmission error.

Key issue: The claim uses “all errors,” which conflicts with the documented intent and practical behavior of ICMP error reporting (focused on specific IP/network delivery failures).

(ii) “A redirect message is used when a router notices that a packet seems to have been routed wrongly.”

This is valid. An ICMP redirect is sent by a router to inform the sender/host that it should use a different router (next hop) for reaching a particular destination for subsequent packets.2

So the “routed wrongly / should use another path” intuition matches the redirect purpose, though the more precise phrasing is: a better next hop is available and the host should adjust its routing cache accordingly.2

(iii) “It informs routers when an incorrect path has been taken.”

This is not valid as written. The ICMP Redirect mechanism is described as a router notifying the host that it should change where it sends traffic; it is not an ICMP mechanism whose normal job is to tell routers generally that another router took an incorrect path.2

(iv) “The ‘destination unreachable’ type message is used when a router cannot locate the destination.”

This is valid. “Destination Unreachable” (ICMP type 3 in IPv4) indicates the destination host/network/port is unreachable (including cases where an intermediate router cannot find a path to forward).2

Footnotes

  1. What is ICMP (Internet Control Message Protocol)? - ClouDNS Blog - Discusses ICMP limitations and that it is used for specific error reporting/diagnostics (not all errors). ↩ ↩2

  2. Understand ICMP Redirect Messages - Cisco - Explains redirect behavior and conditions, referencing RFC 792. ↩ ↩2 ↩3

  3. ICMP Redirect Attack Lab - SEED Labs (PDF) - States redirects inform the sender about a different router for subsequent packets and are sent by routers to the sender. ↩ ↩2 ↩3

  4. ICMP Message Definitions for Isolating Network Connectivity Issues - Includes definition for “Destination Unreachable”. ↩

  5. Internet Control Message Protocol (ICMP) - LiveAction - Describes Destination Unreachable and its troubleshooting meaning. ↩

Validity of Each Statement w.r.t. ICMP

Based on ICMP type semantics and delivery scope

Common confusion points

Pro Tip: Use ‘who sends’ and ‘who receives’

When evaluating ICMP statements, check the direction: Redirect is a router-to-host optimization message, and Destination Unreachable is an error response tied to reachability failures to the sender.3

Footnotes

  1. Understand ICMP Redirect Messages - Cisco - Explains redirect behavior and conditions, referencing RFC 792. ↩

  2. ICMP Redirect Attack Lab - SEED Labs (PDF) - States redirects inform the sender about a different router for subsequent packets and are sent by routers to the sender. ↩

  3. ICMP Message Definitions for Isolating Network Connectivity Issues - Includes definition for “Destination Unreachable”. ↩

Warning: Avoid ‘all’ and ‘always’ in ICMP semantics

Questions often include overly broad claims (e.g., ‘reports all errors’). ICMP error reporting targets specific IP/network-layer delivery issues, not every possible transmission error.

Footnotes

  1. What is ICMP (Internet Control Message Protocol)? - ClouDNS Blog - Discusses ICMP limitations and that it is used for specific error reporting/diagnostics (not all errors). ↩

Knowledge Check

Question 1 of 4
Q1Single choice

Which ICMP message type is used when the destination host/network/port is unreachable?

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