Ping can also be used to test the ability of a local host to communicate across an internetwork. The local host can ping an operational IPv4 host of a remote network, as shown in Figure 29-8. The router uses its IP routing table to forward the packets.
Figure 29-8 Testing Connectivity to a Remote LAN
Go to the online course to view an animation of pinging a device on a remote LAN.
If this ping is successful, the operation of a large piece of the internetwork can be verified. A successful ping across the internetwork confirms communication on the local network, the operation of the router serving as the default gateway, and the operation of all other routers that might be in the path between the local network and the network of the remote host.
Additionally, the functionality of the remote host can be verified. If the remote host could not communicate outside of its local network, it would not have responded.
Note
Many network administrators limit or prohibit the entry of ICMP messages into the corporate network; therefore, the lack of a ping response could be due to security restrictions.
Traceroute—Test the Path (29.2.5)
Ping is used to test connectivity between two hosts but does not provide information about the details of devices between the hosts. The traceroute (tracert) utility is used to generate a list of hops that were successfully reached along the path. This list can provide important verification and troubleshooting information. If the data reaches the destination, the trace lists the interface of every router in the path between the hosts. If the data fails at some hop along the way, the address of the last router that responded to the trace provides an indication of where the problem or security restrictions are found.
Using traceroute provides the round-trip time for each hop along the path and indicates if a hop fails to respond. The round-trip time is the time a packet takes to reach the remote host and for the response from the host to return. An asterisk (*) is used to indicate a lost or unreplied packet.
This information can be used to locate a problematic router in the path, or it may indicate that the router is configured not to reply. If the display shows high response times or data losses from a particular hop, this is an indication that the resources of the router or its connections might be stressed.
Traceroute makes use of a function of the TTL field in IPv4 and the Hop Limit field in IPv6 in the Layer 3 headers, along with the ICMP Time Exceeded message.
The first sequence of messages sent from traceroute have a TTL field value of 1. This causes the TTL to time out the IPv4 packet at the first router. This router then responds with an ICMPv4 Time Exceeded message. Traceroute now has the address of the first hop.
Traceroute then progressively increments the TTL field (2, 3, 4, and so on) for each sequence of messages. This provides the trace with the address of each hop as the packets time out further down the path. The TTL field continues to be increased until the destination is reached or a predefined maximum TTL value is reached.
After the final destination is reached, the host responds with either an ICMP Port Unreachable message or an ICMP Echo Reply message instead of the ICMP Time Exceeded message.
In Figure 29-9, the host at 10.0.0.1 has sent three ICMPv4 messages. The next ICMPv4 message sent by that host will reach 192.168.1.2. The host at 192.168.1.2 will respond with an ICMP Echo Reply message.
Figure 29-9 Tracing the Route to a Destination
Go to the online course to view an animation of Figure 29-9.
Packet Tracer—Verify IPv4 and IPv6 Addressing (29.2.6)
IPv4 and IPv6 can coexist on the same network. From the command prompt of a PC, there are some differences in the way commands are issued and in the way output is displayed.
Refer to the online course to complete this Packet Tracer.
Packet Tracer—Use Ping and Traceroute to Test Network Connectivity (29.2.7)
There are connectivity issues in this activity. In addition to gathering and documenting information about the network, you will locate the problems and implement acceptable solutions to restore connectivity.
Refer to the online course to complete this Packet Tracer.
Packet Tracer—Use ICMP to Test and Correct Network Connectivity (29.3.1)
In this Packet Tracer activity, you will use ICMP to test network connectivity and locate network problems. You will also correct simple configuration issues and restore connectivity to the network.
• Use ICMP to locate connectivity issues.
• Configure network devices to correct connectivity issues.
Refer to the online course to complete this Packet Tracer.