全世界的极客都知道他们的本地主机是127.0.0.1，但是为什么在所有可用的地址中，这个特定的地址是为本地主机保留的呢？请继续阅读，深入了解本地主机的历史。

GMPhoenix提供的图片；可在此处作为墙纸使用。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，它是一个由Q&a网站组成的社区驱动分组。

问题

超级用户读者Roee Adler对默认的本地主机IP很好奇，他向社区提出了以下问题：

I wondered what is the origin of the decision to make localhost‘s IP address 127.0.0.1. What is the “meaning” of 127? what is the “meaning” of 0.0.1?

到底是什么意思？虽然不知道这些问题的答案就有可能活出你整个古怪的生活，但我们已经准备好深入研究了。

答案

几个贡献者参与回答了Roee的问题，他们的每一个贡献都有助于更清楚地说明127.0.0.1是我们称之为家的地方。约翰T写道：

127 is the last network number in a class A network with a subnet mask of 255.0.0.0. 127.0.0.1 is the first assignable address in the subnet. 127.0.0.0 cannot be used because that would be the wire number. But using any other numbers for the host portion should work fine and revert to using127.0.0.1. You can try it yourself by pinging 127.1.1.1 if you’d like. Why they waited until the last network number to implement this? I don’t think it’s documented.

Hyperslug通过挖掘关于这个主题的旧备忘录来进行一些档案调查：

Earliest mention I can find regarding 127’s assignment as loopback is November 1986 RFC 990 authored by Reynolds and Postel:

The address zero is to be interpreted as meaning “this”, as in “this network”.

For example, the address 0.0.0.37 could be interpreted as meaning host 37 on this network.

…

The class A network number 127 is assigned the “loopback” function, that is, a datagram sent by a higher level protocol to a network 127 address should loop back inside the host. No datagram “sent” to a network 127 address should ever appear on any network anywhere.

Even as early as September 1981 RFC 790, 0 and 127 were already reserved:

0 and 127 were the only reserved Class A networks by 1981. 0 was used for pointing to a specific host, so that left 127 for loopback.

I know this doesn’t answer the question, but this is as far back as I could dig. It might have made more sense to choose 1.0.0.0 for loopback but that was already given to BBN Packet Radio Network.

虽然我们都知道并喜欢127.0.0.1作为localhost，但值得注意的是，它不会永远是localhost。127.0.0.1是如何在IPv4通信中指定本地主机的，随着IPv6的慢慢接管，它将由一个更直观的数字来指定：0:0:0:0:0:0:1。

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