通常，我们只需要输入一个我们想要查看的网站的地址，但是如果我们使用直接IP地址，web服务器真的会“知道”吗？今天的超级用户问答帖子回答了一个困惑读者的问题。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

照片由Cory M.Grenier（Flickr）提供。

问题

超级用户读者Joseph A.想知道web服务器如何判断您是否使用直接IP地址访问：

Some web servers, when accessed using their IP address, return an error that direct IP address access is not allowed. I have been wondering for some time how this works.

Does a browser always resolve the IP address and connect to it? Direct IP address access is just skipping DNS altogether, right? How does a remote server even know that you skipped DNS?

web服务器如何知道您是否使用直接IP地址访问？

答案

超级用户贡献者iAdjunct为我们提供了答案：

To answer your question of how it knows, it has to do with what your browser sends the web server. You are right that the system always resolves it to an IP address, but the browser sends the URL you attempted to access in the HTTP header.

Here is a sample header that I found online, modified to look as though you used Firefox on Windows and typed apple.com into the address bar:

Here is what the header would look like if you used its IP address:

Both of these would be sent to the same IP address over a socket, but the browser tells the web server what it accessed. Why? Because web servers with the same IP address may host multiple websites and give different pages for each. It cannot distinguish who wants which page by IP address because they all have the same one, but it can distinguish them by the HTTP header.

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