If they are not at their office, you then have to call their cell phone number, which is
also provided by another company.
Each of these companies will have an identity for the subscriber that only they know,
for billing purposes. So in reality, a subscriber has multiple identities today for their
device. The identity is not necessarily for the subscriber, but the actual device used to
reach the subscriber.
Imagine now if you could have one number assigned to you as a subscriber, and calls
would find you no matter where you were. The telephone number is now associated
to you the subscriber, rather than the device. All anyone would need to know is your
identity, and the network would take care of routing to the right device based on your
location. This means that the present form of identification must change.
90 Chapter 4
There are several parts to a telephone number, just as there are many different
parts to an IP address. The telephone number ends with the subscriber line identifier,
preceded by the address of the serving switch (or exchange as they were once referred
to). This is preceded by a city code, and a country code. The switches in the network
dissect the telephone number in sections, and use these various portions of the address
to determine how to route the call (based on routing tables that identify the circuit[s]
to use to connect to the next hop in the call).
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