It should be noted that the IP addresses of these entities are static, and therefore
changes made to their addresses are infrequent. This is not the case for subscribers
and their addresses, so we will talk about that separately. So for the purposes of IMS
identities, each identity starts with a domain name, which must be resolved to a physical
address in the network. The physical address is an IP address.
Each network uses its own domain name server (or series of servers if it is a large
network) to resolve the addresses of its own entities. These servers know the addresses
within their domain, but they also have connectivity to the next level of domain servers
outside of the network domain, allowing them to query other servers for addresses
in other networks. There is no one DNS that knows the addresses of every subscriber
in the world.
For example, a network in the U.S. may need to connect to an I-CSCF in Australia.
The network operator probably would not have the IP address of the I-CSCF it needs
to connect with in Australia, but it does know the address of a domain name server
in Australia that would. It would then query this server, which in turn could query
Addressing in the IMS 99
yet another server, or simply forward the request to the domain name server with the
proper database and IP address being queried.
If a subscriber is outside their home network (they are roaming in another network),
the visited network is not responsible for resolving the addresses.
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