Wait, is 9.9.9.10 the secondary? That's in the same allocation as 9.9.9.9. What's the point in having a secondary if there's no real separation or redundancy?
8.8.8.8 and 8.8.4.4 are separate allocations - both /24.
8.8.8.8 and 8.8.4.4 are separate allocations - both /24.
In both networks, those IP addresses are almost certainly treated identically, virtualized to all hell with multiple physical termination points leading to the same pools of machines. One extra /24 isn't going to help reliability much if at all, especially considering it is part of the same AS.
Perhaps I'm wrong and Google use the /24 somehow for the purposes of internal routing. If that's true, in the same scenario IBM may be content to have just these two /32s in their internal tables where route aggregation could be be made to not apply.
The primary IP address for Quad9 is 9.9.9.9, which includes the blocklist, DNSSEC, and other security features. However, there are alternate IP addresses that the service operates which do not have these security features. These might be useful for testing validation, or to determine if there are false positives in the Quad9 system.
Secure IP: 9.9.9.9 Blocklist, DNSSEC, No EDNS Client-Subnet
Unsecure IP: 9.9.9.10 No blocklist, no DNSSEC, send EDNS Client-Subnet
Note: Use only one of these two addresses. Some networking software may include terminology such as “Secondary DNS Server” in configuration windows; this can be left blank. Putting both 9.9.9.9 and 9.9.9.10 into “primary” and “secondary” fields may result in unsecure results in rare circumstances.
9.9.9.10 does not have blocking, it also has edns enabled (so assume less privacy than 9.9.9.9 given edns will be transmitted on 9.9.9.10)
We will make sure to push out some documentation on the different services on each ip. The launch we focused on 9.9.9.9. There will be a blocking +edns ip soon we just dodnt get it done in time. :(