ersal key that is globally accessible by all potential correspondents of the user. With user ENUM, the data is public. Anyone who knows the universal key-that is, the phone number-can have access to the information, which may have privacy implications. Even if the user subscribes to several applications and services, the user would remain the only party who has complete control over the set of identifiers. User ENUM is a capability pro- vided for end users and is optional both for the calling user and for the called user. Infrastructure ENUM-Network operators using IP-based technology within their networks cannot rely on an optional technology used by end users. They need an independent routing mechanism to find the ingress points to their networks. Using DNS and ENUM technology for this pur- pose, as described in RFC 3761, is called infrastructure ENUM, or carrier, or operator, ENUM. The basic premise of infrastructure ENUM is to provide information only to IP service providers and, in some cases, only to selected peers. The end user typically cannot use this information or has no access to it. This type of system must be implemented as an independent system. If every IP communications service provider is only providing data for numbers hosted by the operator itself, a later merging of trees should not be a problem. If, on the other hand, providers are entering data for numbers they are not hosting themselves (e.g., data for numbers where they provide transit services), a later merging of trees will cause problems. Therefore, it Chapter 9 IP Services is not recommended to use infrastructure ENUM for providing transit information. Another application for infrastructure ENUM technology is to provide access to national number portability information, which is currently stored in databases. However, this information will have only national significance (e.g., national routing numbers). As a result, this data cannot be used in supranational infrastructure ENUM implementations. The reality is that ENUM is not currentl