If a network administrator sees an IP address connecting only after production hours, which tool should they use for investigation?

The most appropriate tool for investigating an IP address that connects only after production hours would be nslookup. This utility is typically utilized for querying the Domain Name System (DNS) to obtain domain name or IP address mapping information. By using nslookup, the network administrator can gather valuable details about the IP address in question, such as its domain name, which can help identify the source of the connection and determine whether it is legitimate or potentially malicious.

Other tools serve different purposes: ifconfig is used for configuring and displaying network interface parameters on Unix-like operating systems; nmap is a network scanning tool used to discover hosts or services on a computer network; ssh (Secure Shell) is a protocol for secure network services, primarily for accessing remote machines. Thus, while each of these tools has its value, nslookup specifically aids in understanding and investigating the DNS records associated with the suspicious IP address.