S1 / 13 MODULE 13 / ICMP AND CONNECTIVITY TESTING
Test the network.
Interpret the response.
ICMP reports network-layer errors and supports diagnostics. Ping and traceroute use ICMP behavior to help identify whether a destination or path is reachable.
Understand ICMP messages
ICMP for IPv4 and ICMPv6 for IPv6 carry control and error-reporting messages. Examples include Destination Unreachable and Time Exceeded. Common ICMPv4 Destination Unreachable codes include 0 network, 1 host, 2 protocol, and 3 port unreachable; ICMPv6 uses different code values. ICMP is not an application transport protocol like TCP or UDP; it supports IP operation and diagnostics. ICMP feedback may be filtered, rate-limited, or suppressed, so the absence of an ICMP response does not prove a device is down.
IMG-1010 · IMG-0998 · IMG-1009 · IMG-1011 · IMG-1019Use ping to test reachability
Ping sends an Echo Request and waits for an Echo Reply. A reply confirms that the destination responded to ICMP at that time; it does not prove that a specific application or TCP/UDP port is available. A timeout can indicate a routing or addressing problem, but it can also mean that a firewall or host policy filters ICMP. The first ping may time out while ARP or IPv6 Neighbor Discovery resolves the local next-hop address; compare repeated probes and inspect the neighbor cache before declaring a fault. Interpret ping alongside interface state and routing information.
IMG-1028 · IMG-1001Use traceroute to inspect a path
Traceroute sends probes with progressively increasing TTL (IPv4) or Hop Limit (IPv6). Each router decrements the value; when it reaches zero, the router may return a Time Exceeded message. The sequence of responses helps reveal the path and where forwarding stops. An asterisk means no response arrived before the timeout; later hops may still respond, so a silent hop is not proof of a forwarding failure. Probe types vary by implementation: Windows tracert commonly uses ICMP Echo probes, while common Unix/Linux and Cisco IOS traceroute implementations traditionally use UDP probes; other tools can use ICMP or TCP. The shared mechanism is increasing TTL/Hop Limit and intermediate Time Exceeded responses. On a Cisco router, the default probe source is normally the outgoing interface address unless another source is selected.
IMG-1028 · IMG-2271Use a layered connectivity test sequence
Start with the local protocol stack (127.0.0.1 or ::1), then test the host address and local interface, then the default gateway, then a remote IP address, and finally the application by its actual protocol and port. If loopback succeeds but the gateway fails, investigate host addressing, VLAN, link state, ARP/Neighbor Discovery, and the gateway interface. If the gateway succeeds but a remote destination fails, inspect routing, ACLs, and the return path. A successful ping is one piece of evidence, not proof that every service works.
Loopback → Local interface → Gateway → Remote IP → ApplicationCHECK YOUR UNDERSTANDING
ICMP
Practice the key terms, procedures, and troubleshooting decisions from this module.