A technician is unable to join a computer to the Windows domain.

If the PC can ping the domain controller by IP, basic connectivity exists. Domain join, however, depends heavily on name resolution (finding the domain and locating domain services), so incorrect DNS is the most likely cause. Mike Meyers explains a key diagnostic pattern: when you ping a URL/name and get “could not find host,” the system “can’t get an IP address for that Web site,” which “points to a DNS failure.” This same logic applies to domains: if the client can reach the DC by IP but can’t properly resolve domain names/services, the join fails.

Quentin Docter reinforces that one of the “key things” for TCP/IP to work effectively is that “a hostname must resolve to an IP address—an action usually performed by a DNS server,” and he highlights nslookup as a tool to verify DNS server entries.

Firewall issues could block some traffic, but the clue “ping by IP works” points away from total network blockage. IPv6 enabled is normal, and “not connected” contradicts successful ping. Therefore, DNS settings are incorrect (A).