com. AOL owns the domain and is my e-mail provider. As the
networks evolve, everything you do may be under one domain name.
Still, these must be converted to an IP address so that a connection to the subscriber
can be made. There are two methods used for resolving URIs to an IP address. We will
talk about network entities first, and then subscription identities.
Each of the IMS entities is assigned a SIP URI, just as subscribers are. The network
entities are static, and therefore it is quite easy to automate the function of resolving
98 Chapter 4
URIs to IP addresses. This is accomplished through a rather remarkable database known
as Domain Name System (DNS). DNS is a distributed database, consisting of thousands
of name servers deployed all over the world, in a truly hierarchical network.
The top level of the hierarchy consists of 13 DNS servers, distributed all over the
world. There are 13 of these servers located in the U.S., 1 in Japan, 1 in London, England,
and 1 in Stockholm, Sweden. These servers identify the locations of name servers for
each of the domains. For example, the servers in the U.S. provide name resolution for
all domains located in the U.S. This includes all domains with an extension of .com, .gov,
.edu, .mil, etc. The right-most part of the domain name (.com for example) identifies the
type of entity that owns the domain name.
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