A whois request includes this:
NetRange: 104.16.0.0 - 104.31.255.255
CIDR: 104.16.0.0/12
NetName: CLOUDFLARENET
NetHandle: NET-104-16-0-0-1
Parent: NET104 (NET-104-0-0-0-0)
NetType: Direct Allocation
OriginAS:
Organization: Cloudflare, Inc. (CLOUD14)
RegDate: 2014-03-28
So all of `104.16.0.0/12 is assigned to Cloudflare.
My understanding is that iCloud uses Cloudflare. This is confirmed by my iPhone whose public IP address is 104.28.90.x.
So 104.28.152.x is allocated to Cloudflare. If you are using iCloud Private Relay with an address in that range, it is likely a legitimate iCloud Private Relay address.
AFAIK there is no public list of address ranges used by iCloud Private Relay. Edit: But see answer from @Spotlight which does include reference to such a list.
Right now my Mac is connected to iCloud Private Relay and has an Akamai public IP address in the 172.224.0.0/12 range. So Apple uses both Cloudflare and Akamai.
