Ideally, the browser will simply switch over to the IPv4 protocol and make a connection.

理想情况下，浏览器将仅会切换回IPv 4协议并建立连接。

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Today's Internet was built largely on the IPv4 or Internet Protocol version four, first introduced in 1980.

Curt Hopkins / 2010年5月19日 3:49 PM 今天的互联网主要是基于IPv4，或者叫互联网协议版本4（Internet Protocol version four）建成的，于1980年首次被引入。

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Companies who take advantage of IPSec for IPv4 being a protocol option, not a requirement, can skip this option and replace it with proprietary protocol security solutions.

利用IPSec for IPv4(协议选项而不是要求)的公司可以跳过该选项，并使用私有协议安全解决方案替代。

youdao

The internet was built on version 4 of the Internet Protocol (IPv4) which has an upper limit of about four billion addresses.

BBC: Net approaches address exhaustion

The problem is that the original version of Internet Protocol (IPv4), on which the bulk of the Internet has been built thus far, has a fixed capacity of 4.3 billion IP addresses.

FORBES: The Internet Gets Its First Global Upgrade: Happy World IPv6 Day

This expansion is necessary because the current number of addresses under Internet Protocol version four (IPv4) is gradually being exhausted.

WHITEHOUSE: Office of Science and Technology Policy Blog