关于网络地址转换 (NAT)？
只是对 NAT 的特定场景感到好奇。假设我们有 4 台计算机在 NAT 下共享一个全局 IP 地址。据我了解，NAT 盒会保留内部记录，以了解将请求转发到哪台计算机。但假设我正在 2 号计算机上下载一个文件。假设在计算机 #1、#3 和 #4 上，我只是正常浏览网页。当浏览器启动 TCP 连接来获取该文件时，它如何知道将其提供给哪台计算机？我的意思是，四台计算机中的每一台都使用端口 80 来浏览网页，对吧？ NAT的记录如何区分哪个“80端口”属于哪台计算机呢？
Just curious about a particular scenario of NAT. Let's suppose we have 4 computers sharing a global IP address under the NAT. I understand that the NAT box keeps an internal record to know which computer to forward requests to. But let's say on computer #2 I'm trying to download a file. And let's say on computer #1, #3, and #4, I'm just browsing the web normally. When the browser initiates a TCP connection to get that file, how does it know which computer to give it to? I mean like, each of the four computers is using port 80 to browse the web right? How does the NAT's record distinguish which "port 80" belongs to which computer?
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互联网上每个唯一的 TCP 连接均由四个数字组成 -
{源 IP、源端口、目标 IP、目标端口}。NAT 网关 (GW) 将其转换为
{GW 公共 IP、GW 映射端口、目标 IP、目标端口}，以便外部路由器知道将数据包返回到该特定网关。它还保留这些映射端口到源 IP 和端口号的映射，沿着{GW-mapped port ->; {源IP，源端口}}，这使得它能够确定将响应数据包发送到哪个内部机器。Each unique TCP connection on the internet is made up of four numbers -
{source IP, source port, destination IP, destination port}.A NAT gateway (GW) translates this to
{GW public IP, GW-mapped port, destination IP, destination port}so the outside routers know to return packets to this particular gateway. It also keeps a mapping of these mapped ports back to source IP and port number, along the lines of{GW-mapped port -> {source IP, source port}}, which allows it to figure out what internal machine to send the response packets to.“http 的端口 80”的概念并不像这些那样工作。当计算机浏览网页时，只有服务器使用80端口，而客户端将使用随机端口号。服务器回复附有客户端提供的目标端口。 80端口只是用来敲Web服务器的门的。
NAT 所做的就是将所有这 4 台计算机向外转换数据包，以便它们的源端口不会重复。当NAT收到数据包时，它会检查所连接的目标端口是否可以转换，如果可以的话将其转换到LAN。
The concept of "port 80 for http" does not work like these. When a computer browse the web, only the server uses port 80, while the client will use a random port number. The server replies with a destination port, provided by the client, attached. Port 80 is just for knocking the web server's door.
What the NAT does do is translating all those 4 computers outward packets such that their source ports does not duplicate. When the NAT receives a packet, it will check if the attached destination port can be translated and translate it to the LAN if possible.