我们给定一个具体的场景：

域名：  xyz.com
dns服务器   ns1.xxx.com
将绑定的ip地址  123.123.123.123
apache2安装在  123.123.123.123

在apache2的配置文件/etc/httpd/conf/httpd.conf中，有两个地方会出现ServerName。
第一个地方：

ServerName gives the name and port that the server uses to identify itself.
This can often be determined automatically, but we recommend you specify
it explicitly to prevent problems during startup.
If your host doesn't have a registered DNS name, enter its IP address here.

ServerName localhost:80

这里的ServerName localhost:80 是否可以修改成
ServerName 123.123.123.123:80
我理解成这样，是根据英语
If your host doesn't have a registered DNS name, enter its IP address here.
问题来了：如果我的主机有registered DNS name，应当如何处理呢？

第二个地方
你需要写下面这样的代码

<VirtualHost *:80>
    ServerName www.xyz.com
    DocumentRoot "/var/www/html"
</VirtualHost>

从上面，我们可以看出：都是ServerName，但是他们对应的具体值，不一样。
1。我的理解对吗？
2。两个地方，我都写对了吗？
3。还有个问题：如果我的主机有registered DNS name，应当如何处理呢？