 In this module, we shall look at what are the different forms of identifiers which are used in NGN. Specifically, we'd look at the three types which are adopted in NGN to identify the objects and entities. The identifiers in NGN are defined as the numbering system, the naming system and the addressing system. The numbering system is defined to determine how many devices are there in the network. It is similar to the IP addressing. The naming, you can consider it something similar to the DNS. An addressing is again similar to IP address on the basis of which the packets are delivered. The three types of identifiers defined in NGN are first the host domain name. The host domain name or the domain name in simple words identifies the operator for routing the SIP registration requests to the home operators IP multimedia subsystem. If you recall, in the NGN reference architecture, IMS is a mandatory entity. So for home domain name representation, the DNS format is utilized. For instance, it could be an operator 123.com. The details of the home domain name and its associated additional information are provided in the IP multimedia subsystem subscriber identification module. Then the next important identifier is the private IDs. Whenever a user subscribes to the NGN services, it needs to be given a private ID. Each user needs to have at least a private ID. This private ID is going to stay with the user for as long as the user stays part of the network and it is not dependent upon the session or the duration of the service. A user can have more than one IDs as well for subscribing to different services. The syntax is again, it is the username at the rate something called let's say Realm. Realm is actually the host domain name. For instance, we were talking about the operator 123.com. Likewise, the syntax can be username at the rate 123 operator.com. This information is also stored locally on the IP multimedia subsystem in the subscriber identification module. Now the private ID is network aware. It means whenever the network changes, the private ID of the user also changes because it is associated to the network it belongs to. Then the next identifier is the public ID. Similar to the private ID, each IMS user may also have one or several public IDs. Now the difference between public ID and the private ID is that the public ID is used for routing and forwarding the messages to that particular user. For instance, if the SIP message is to be routed to the end user for call establishment, for call forwarding and for any other important notification, then the public ID is utilized. The format of the public ID can be either in the form of the telephone user uniform resource identifier. For instance, E.164 based, which if you recall is the E.164.ARPA mechanism which is provided by NM. And then we have the SIP URI. The SIP URI is used in especially the voice over IP, dialing and signaling. And the format is again, it is a user at the rate the administrative domain it belongs to. Now again, the public IDs are user aware. It means against every user, the public ID is going to change. To best understand what are public and private IDs, you can establish or try to establish a relationship between the domain name and the IP address. But here in NGN, the properly designated terminologies are the public and private IDs.