It is only visible to network operators that assign them. There are two
different types of user identities; a public user identity and a private user identity.
Private User Identity The private user identity is assigned by the home service provider.
This identity is used for registration of the subscriber, authentication, and authorization,
administration of the subscription, and of course billing and accounting. The
private user identity is never used for routing. It is embedded within the device itself,
either through some administrative effort or some automated function, dependent on
implementation.
For example, earlier in this chapter we discussed the use of SIM cards within a device.
Regardless of whether the device is a wireline or wireless device, the SIM card
would be inserted into the device. The SIM card is programmed by the service provider
with the private user identity for the subscription. Think of the private user identity
as the identity for a subscription.
If there is no private user identity assigned in the case of wireless, then the International
Mobile Subscriber Identifier (IMSI) is used to create the private user identity. The private
user identity would take the form of
@mnc.mcc.3gppnetwork.org.
The first part of this address is the IMSI itself, followed by the mobile national code
(MNC), and the mobile city code (MCC).
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