Addressability is the ability of a digital device to individually respond to a message sent to many similar devices. wikipedia 
Examples include pagers, mobile phones and set-top boxes for pay TV. Computer networks are also addressable, such as via the MAC address on Ethernet network cards, and similar networking protocols like Bluetooth. This allows data to be sent in cases where it is impractical (or impossible, such as with wireless devices) to control exactly where or to which devices the message is physically sent.
In the case of simple hardware devices like the pager, the address is simply the electronic serial number (and later IMEI/MEID) in its firmware, or physically manufactured into its circuitry. In the case of GSM mobile phones, it also includes the subscriber identity module, which is also present as a smart card on satellite TV receivers, or a different PCMCIA CableCARD for cable TV. Addressing and encryption are used together for conditional access to different TV channel bundles which a pay-TV customer has or has not paid for.
Addressing is also done in software at higher levels such as IP addresses, which can be dynamically allocated. Even physically separate devices are now addressable, such as to enforce revocation lists for digital restrictions, or to use the former DIVX DVD video rentals (although the latter only used its identity to "phone home" for billing purposes).
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