Figure 131 Connectivity Testing Flowchart

The first step is to attempt to ping the target host by its host name. If this succeeds, it validates that everything from name resolution to physical delivery of the data is functioning properly.

If this is not successful, the next step is to attempt to ping the target host by its

IP address. This eliminates name resolution as part of the problem. If this succeeds, the problem is likely going to be related to name resolution (either Domain Name System [DNS] resolution or NetBIOS name resolution). Perhaps the DNS server is down or the target host name is not known. If this does not succeed, it is possible that the target host is inaccessible for some reason (regardless, however, the problem warrants more attention).

If the target host is remote, the next step is to attempt to ping the default gateway of the source machine as well as another host on the same network as the remote machine (it is a good idea to use the remote machines default gateway as the destination). If you can physically access the target machine, repeat this process in the other direction. These steps validate that both hosts are able to communicate with their local routers as well as validating that you can reach something on the remote network. If they cannot, a good chance exists that the problem exists between the host and its router. It is possible that there is an invalid Address Resolution Protocol (ARP) entry on either the host or the router.

If the hosts can successfully ping their respective routers, and you are unable to ping another host on the remote network, the next step is to perform a traceroute from the source to the target host. This will enable you to determine the approximate location where the problem is occurring. In a complex routed environment, it is generally a good idea to have some baseline results of a functional traceroute because the traceroute is typically unable to provide the IP address of the failing hop. Only by knowing what the next hop from the last successful hop is can you have an idea of what specific router might be the cause.

Testing Connectivity Without Using Ping

One thing to keep in mind when testing connectivity is that many firewalls, by design, do not allow ICMP traffic to traverse the firewall, and thus render the use of ICMP to test connectivity worthless. You have a couple of options in this event. For one, you can permit the ICMP traffic for the purposes of troubleshooting the problem and then disable it again when you are finished. Another option is to use another protocol to determine whether the remote system is responding at all. For example, just telnetting to many TCP ports will either confirm or deny whether a remote host is accessible, as shown in Example 13-1.

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