# 主域名与子域名的区别

**主域名**

也称为第1级域名，只有一个点(句点)，如`「mysite.com」`([co.jp](/cn/faq/tag-and-domain/domain-main-vs-sub.md)、ne.jp等情况下有两个)。子域名位于此域名之前。

**子域名**

位于主域名之前的域名，其中含有多个点。例如，`「domain.mysite.com」`是第2级子域名。

**第3级子域名**

在"域名"之前添加新内容即可，如`"help.domain.mysite.com"`。N级子域名在N-1级子域名之前添加。

**目录**

类似于显示计算机位置的文件夹，是路径字符串。

`"mysite.com"`通常被视为网站的根目录，子目录位于根目录下，链接形式通常指定为在域名末尾添加"/"的形式，如`www.mysite.com/agent`。"/"是子目录。

**注意：** 在Ptengine中，只需添加主域名，就可以测量主域名下的所有子域名和子目录。


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