Question: In an IMS (IP Multimedia Subsystem) network, what is a requirement for IMS nodes performing ENUM/DNS resolution to accommodate varying top-level domains for ENUM domain names?
Option 1: Mandatory use of 'e164.arpa' as the top-level domain
Option 2: Configuration option for the top-level domain set by the network operator
Option 3: Automatic detection of the top-level domain based on the querying IMS operator
Option 4: Use of a default top-level domain predefined by 3GPP specifications
Explanation: This is correct because, according to the specified context, the top-level domain for ENUM domain names (which may differ from 'e164.arpa') should be configurable by the network operator in all IMS nodes capable of performing ENUM/DNS resolution. This flexibility accommodates varying operational requirements and scenarios where the universal ENUM service's top-level domain might not be used.