Which ICMP Message Comes Back?

About this interactive

The three messages, with their type numbers (RFC 792 for IPv4, RFC 4443 for ICMPv6): - **Echo reply** (IPv4 type 0, ICMPv6 type 129): the answer to an echo request (type 8 or 128), which is what ping sends. - **Destination unreachable** (IPv4 type 3, ICMPv6 type 1): the packet cannot be delivered. Its code says why: network or host unreachable, protocol unreachable, port unreachable, administratively prohibited by a filter; in ICMPv6, no route, administratively prohibited, beyond scope of source address, address unreachable, port unreachable. - **Time exceeded** (IPv4 type 11, ICMPv6 type 3): a router lowered the TTL (IPv4) or hop limit (IPv6) to zero and dropped the packet. Traceroute sends probes with TTL 1, 2, 3 and so on, and each router that answers with time exceeded is one hop on the path. The video says an expired TTL comes back as "destination unreachable". It does not: it is time exceeded, a different message type.

What you'll learn

Predict the ICMP message a sender gets back: an echo reply, a destination unreachable (no route, a host or port that cannot be reached, a protocol not run, a firewall refusing) or a time exceeded
Explain how traceroute uses the TTL or hop limit: each router where it reaches zero returns a time exceeded, which maps the path hop by hop

Aligned to

CompTIA Network+

1.4 Explain common networking ports, protocols, services, and traffic types.

5.5 Given a scenario, use the appropriate tool or protocol to solve networking issues.

Cisco CCST Networking

5.3 Run basic diagnostic commands and interpret the results

Cisco CCNA

1.8 Configure and verify IPv6 addressing and prefix

Key terms

Internet Control Message Protocol: A network layer protocol used to send error messages and operational information about network conditions.
Echo Request: An ICMP message sent by a source device to test reachability of a destination host, commonly used by the ping utility.
Echo Reply: An ICMP message returned by a destination host in response to an echo request, confirming reachability and providing round-trip timing information.
Time to Live: A value in a packet that limits its lifespan on a network, preventing it from circulating indefinitely.
Hop Limit: A field in the IPv6 header that starts at a value set by the sender and is decremented by one at each router; when the value reaches zero the packet is discarded and an ICMPv6 error is sent to the source. The hop limit serves the same loop-prevention purpose as the TTL field in IPv4.
traceroute: A Linux command-line tool that maps the path packets take to a destination host by displaying each intermediate hop and its latency.

Topics

Icmp Ping Traceroute Ttl Destination Unreachable Time Exceeded Interactive Categorize

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