CIDR for Dummies DBA in Cloud

For DBAs of Cloud, its imperative to learn various networking concepts and CIDR is one of them. Without going into much detail, I will just post here quick note as what CIDR is and how to use it.



A CIDR looks something like this:

10.0.0.0/28

The 10.0.0.0/28 represents range of IP addresses, and no its NOT from 10.0.0.0 to 10.0.0.28. Here is what it is:

So in order to know how many IP address are in that IP range and from where it starts and where it ends, the formula is :

2 ^ (32 - )

So for the CIDR 10.0.0.0/28 :

2 ^ (32 - 28) = 2 ^ 4 = 2 * 2 * 2* 2 = 16

So in CIDR range 10.0.0.0/28 , we have 16 IP addresses in which

Start IP = 10.0.0.0
End IP  = 10.0.0.15



Also cloud providers normally reserve few IPs out of this CIDR range for different services like DNS, NAT etc. For example, AWS reserves first 4 and last IP of any CIDR range. So in our example , we would just have 10 IP addresses to work with in AWS.

So in case of AWS, we would have a region where we would have a VPC. CIDR is assigned to that VPC. In that VPC, for example we would have 2 subnets. We can distribute our 10 IPs from our CIDR 10.0.0.0/28 to our both subnets. Below I am giving 5 IPs to each subnet. A subnet is just a logical separate network.

For example we can give:

Subnet 1:

10.0.0.5 to 10.0.0.9

Subnet 2:

10.0.0.10 to 10.0.0.14 

Hope that helps.

PS. And oh CIDR stands for Classless Inter-Domain Routing (or Supernetting)