Throttle IPv6 signup for subnet (#17588)

config/initializers/rack_attack.rb

@@ -82,7 +82,9 @@ class Rack::Attack
   end
 
   throttle('throttle_sign_up_attempts/ip', limit: 25, period: 5.minutes) do |req|
-    req.remote_ip if req.post? && req.path == '/auth'
+    return unless req.post? && req.path == '/auth'
+    return req.remote_ip.mask(64) if req.remote_ip.ipv6?
+    req.remote_ip
   end
 
   throttle('throttle_password_resets/ip', limit: 25, period: 5.minutes) do |req|