Recently, someone preparing for the Oracle Cloud Infrastructure Networking Professional Certification approached me with a query:
Which of the following RFC 1918 CIDR prefixes can be used to create a Virtual Cloud Network (VCN)?
- 172.16. 0.0/12
- 8.8.8.8/8
- 10.0. 0.0/8
- 189.215.154.89/32
- 192.168. 0.0/16
- 0.0.0.0/0
From the provided options the only correct answer is 192.168.0.0/16. And this is pretty obvious that 8.8.8.8/8 and 189.215.154.89/32 are incorrect options. However, he was puzzled as to why the prefixes 10.0.0.0 and 172.16.0.0 were considered incorrect, given that they are supposed to be part of the reserved ranges for private IP addresses.
Indeed, they are. And Oracle recommends using the private IP address ranges specified in RFC 1918, which reserves and recommends the following three blocks of the IP address space for private internet addresses:
- 10.0.0.0/8
- 172.16.0.0/12
- 192.168.0.0/16
The issue here lies in the range.
OCI Private VCNs only supports sizes ranging from /16 through /30.
Therefore:
- 10.0.0.0/16 is used instead of 10.0.0.0/8.
- 172.16.0.0/16 is used instead of 172.16.0.0/12.
- 192.168.0.0/16 is supported.
Let’s wrap up with a piece of advice.
It’s important to note that having two overlapping CIDR ranges within an Oracle Cloud Infrastructure (OCI) Virtual Cloud Network (VCN) is not considered a good practice. Overlapping CIDR ranges can lead to network communication issues and routing conflicts, potentially causing disruptions in connectivity and overall network performance. It’s advisable to carefully plan and allocate non-overlapping CIDR ranges for different VCNs or subnets to ensure efficient network management and seamless communication within the OCI environment.