When there are any issues with an end application such as a web service or mail service, the first troubleshooting step using the bottom-up approach is to verify the data-link layer. Once we have verified the data-link layer using the steps defined in the previous section, the next step is to verify the network connectivity between the end points. Network connectivity between end points can be verified by commonly used fault detection and isolation utility tools, such as Ping and Traceroute.