我的 ufw 状态输出如下所示：
To Action From
-- ------ ----
22 ALLOW Anywhere
80/tcp ALLOW Anywhere
443/tcp ALLOW Anywhere
25/tcp ALLOW Anywhere
22 (v6) ALLOW Anywhere (v6)
80/tcp (v6) ALLOW Anywhere (v6)
443/tcp (v6) ALLOW Anywhere (v6)
25/tcp (v6) ALLOW Anywhere (v6)
为什么他添加了两次规则，v6 代表什么？