The default is:
<system.net>
<settings>
<ipv6 enabled="true" />
</settings>
Tried following, but it stopped Extension Manager from working behind Proxy.
<system.net>
<settings>
<ipv6 enabled="true" />
<servicePointManager expect100Continue="false" />
</settings>
<defaultProxy
enabled="true"
useDefaultCredentials="true">
<proxy usesystemdefault="True" />
</defaultProxy>-->
</system.net>