In one of our production VPCs, we hit a major roadblock — we had completely exhausted the available IP addresses.
Migrating everything to a new VPC with a larger CIDR block was the obvious long-term solution, but The process was highly complex, involving dozens of dependent resources and Our deadline was tight, and a full migration simply wasn’t feasible within the given timeframe.
While researching alternatives and digging through AWS documentation, we discovered a lesser-known yet powerful option: ➤ You can actually add secondary CIDR blocks to an existing VPC.
This was a revelation to us — we always believed CIDRs were immutable once a VPC was created.
Using this approach, we:
Added a new CIDR block to the existing VPC.
Created new subnets from the added CIDR range.
Updated route tables accordingly to ensure proper traffic flow.
The result? We got the additional IP space we needed without the overhead of a full migration — a huge win under pressure!
Here I’m giving you the process I followed.
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Let’s assume, I currently have a VPC configured with the CIDR block 10.0.1.0/28, which provides me with a total of 11 usable IP addresses.
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VPC before adding the new CIDR
I want to run 11 machines within this VPC, which means there isn’t a 12th IP address available if I want to add another instance. As shown below, I added a new CIDR block, 10.0.2.0/28, to my VPC, which provides an additional 11 fresh IP addresses for use.
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New instance spinned up in with IP of newer CIDR range
I completed this task about 18 months ago when I was still a beginner with AWS, and my manager assigned it to me.
Got any questions? Drop a comment — happy to help!
