Is Really Simple SSL suitable for my multisite installation?

Yes, you can use Really Simple SSL for multisite.

When you activate Really Simple SSL on a multisite environment, you can choose to network activate, or to activate per-site.

When you network activate, you get the same option again (might be confusing) but even on network activation, you can choose to enable SSL per site. After activation you’ll be asked to activate per site or network wide.

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