What is the MOST likely issue if an application server is not accessible by hostname but is accessible by IP address?

When an application server is accessible via IP address but not by hostname, the underlying issue is typically related to the way domain names are resolved to their corresponding IP addresses. In this scenario, if a hostname cannot be resolved, it suggests that the DNS (Domain Name System) is not correctly translating that hostname into the appropriate IP address.

A missing DNS resource record, specifically the 'A' record, which associates a hostname with its respective IP address, would result in queries for that hostname not returning the expected outcome. Without this record, clients querying for the hostname would be unable to resolve it to the server’s IP address, thus making the application server inaccessible by its hostname.

Other choices involve different DNS issues. If the forward lookup zone is missing, it would prevent hostnames from being resolved, but the correct issue specifically points towards the absence of the relevant record needed to resolve that hostname. In contrast, the MX record pertains specifically to mail exchanges and is unrelated to general hostname resolution for application servers. The reverse lookup zone focuses on resolving IP addresses back to hostnames, which is not the problem in this case because the IP address is accessible.