From LQWiki
IPv4, stands for Internet Protocol Version 4, which is the current standard protocol for the Internet. IPv4 was devised before anyone had any inkling how popular the Internet would become. Thus the addressing scheme seems not to have been geared to scale with the expansion of the Internet (Though this may be a myth). IPv4 uses 32-bit addresses, which amounts to 4,294,967,296 unique ones, however many are designated for special purposes hence reducing availability. Since it is believed that IPv4 will run out of valid addresses, there are plans to eventually replace IPv4 with IPv6, but IPv6 is not being widely embraced yet.
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