AWS VPC Create Subnet in with different zone

Question

I already have a VPC (Non Default) configured which is used by Amazon EC2.

I need to create another subnet (Non Default) in the same VPC with a different Availability Zone.

So when I am trying to Add details, I am stuck at the CIDR Block. I am getting 2 errors when trying to set different IPs for CIDR.

1. CIDR block 192.168.250.225/16 overlaps with pre-existing CIDR block 192.168.0.0/16 from subnet-6afacb2c (192.168.0.0/16) | AvtarSingh Public Subnet.

2. 192.167.250.225/16 is not within the range of 192.168.0.0/16

I tried many combinations that I was able to think, but none of them are working for me.

I am sorry but I don't have much knowledge of CIDR. So can you give me the correct CIDR for current configuration ? Right now, I just want to set the development environment so want to check.

Following is current VPC & Subnet details:

1. VPC

   VPC ID: vpc-8fe804ea | AvtarSingh VPC
   Network ACL: acl-406a8725
   State: available
   Tenancy: Default
   VPC CIDR: 192.168.0.0/16
   DNS Resolution: yes
   DHCP Options Set: dopt-0d6a7c6f
   DNS Hostnames: yes
   Route Table: rtb-18739f7d
   

2. Subnet

   Subnet ID: subnet-6afacb2c | AvtarSingh Public Subnet
   Availability Zone: us-west-2c
   CIDR: 192.168.0.0/16
   Route Table: rtb-1b739f7e
   State: available
   Network ACL: acl-406a8725
   VPC: vpc-8fe804ea (192.168.0.0/16) | AvtarSingh VPC
   Default Subnet: no
   Available IPs: 65530
   

Answer 1

So basically you created your subnet in the 192.168.0.0/16 range which also means a subnet mask of 255.255.0.0 Your address range for this block is 192.168.0.0-192.168.255.255 so you can use any range in that block for your CIDR block.

So a CIDR block of 192.168.250.225/16 will also have the same address range of 192.168.0.0/16 because you are using the same subnet mask 255.255.0.0 (that's what the /16 means at the end of the address).

You can read more about it here: http://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing

What you want is a CIDR block that is within the 192.168.0.0/16 range, so blocks like these will work for example: 192.168.0.0/24, 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24, 192.168.128.0/24, (these blocks have 256 possible addresses - broadcast and gateway) 192.168.20.0/23 (This block has 512 possible addresses - broadcast and gateway). So in essence, anything that is within the 192.168.0.0/16 and doesn't overlap with an existing CIDR block.

Hope this helps.

Answer 2

You need to edit the CIDR for your VPC and add another CIDR block, say - 192.169.0.0/16. Then navigate to subnets and create the subnet with that range as below :

Answer 3

Your cidr and your VPC are the exact same size.

You need to delete your first subnet so that you can re-cut into multiple subnets.

Rico was leading you down the right path with his /24 examples.

Another good tip is to use a CIDR calculator when making your subnets to see how many address you are going to take up when declaring the new subnet.

One of the simpler ones to use in in my opionin is here