Question: In a 3GPP network, when deriving a Private User Identity from a subscriber's IMSI for an MSC Server enhanced for IP Multimedia Subsystem (IMS) services, how is the realm part of the 'username@realm' format constructed?
Option 1: By concatenating the entire IMSI with a predefined string '.3gppnetwork.org'
Option 2: Using only the Mobile Subscriber Identity (MSIN) part of the IMSI to form a unique domain name
Option 3: Converting the whole IMSI into a numerical domain name without separating MCC and MNC
Option 4: By converting the leading digits (MCC and MNC) of the IMSI into a domain name in the format 'ics.mnc<MNC>.mcc<MCC>.3gppnetwork.org'
Explanation: This is correct because, according to the specified process for deriving a Private User Identity from an IMSI, the realm part of the NAI ('username@realm') is specifically constructed by converting the Mobile Country Code (MCC) and Mobile Network Code (MNC) portions of the IMSI into a domain name format. This ensures a structured and standardized approach to identifying the network (via MCC and MNC) in the Private User Identity, which is crucial for routing and authentication purposes within the IMS architecture.