By using AWS re:Post, you agree to the AWS re:Post Terms of Use
AWS re:Postre:Post

The CIDR 'nnnn' conflicts with another subnet

0

I have spent hours trying to figure out how to solve the CIDR conflicting with another subnet.

As an example, here is a sample CIDR block for a IPv6 CIDRs VPC: 2a05:d01c:f8f:9100::/56

I want to create three subnets for this one VPC.

The first subnet is assigned an IPv6 CIDR block of 2a05:d01c:f8f:9100::/56 with no problems.

The problem arises when I go to create the second subnet, where I am unable to assign a IPv6 CIDR block to I as I keep getting this error "IPv6 CIDR Address overlaps with existing Subnet CIDR: 2a05:d01c:1a0:1f00::/56."

I have tried other IPV6 CDIR addresses, like 2a05:d01c:f8f:9100::/64 or 2a05:d01c:f8f:9101::/56 but the same problem occurs. I am new to CDIR blocks, subnets, VPCs, etc, but have done hours of research and trial and error but still getting nowhere with it.

Please help (:

Thanks

Topics
Networking & Content Delivery
Tags
Amazon VPCIPv6
Language
English
em
asked a month ago186 views
1 Answer
1
Accepted Answer

Hello.

Since the VPC CIDR is "2a05:d01c:f8f:9100::/56", only the range "2a05:d01c:f8f:9100::/56" can be allocated in the subnet.
In other words, if you have already assigned "2a05:d01c:f8f:9100::/56" to the subnet, you will not be able to create another subnet.
For example, if you set the IPv6 CIDR of the first subnet to "2a05:d01c:f8f:9100::/64", you can assign "2a05:d01c:f8f:9101::/64" etc. to the second subnet.
If you calculate with "/64", the prefix will be the first "2a05:d01c:f8f:9100".

The CIDR set for the VPC is "/56", so it looks like the table below.
"2a05:d01c:f8f:9100" to "2a05:d01c:f8f:91ff" can be set by the user.
https://docs.aws.amazon.com/vpc/latest/userguide/subnet-sizing.html#subnet-sizing-ipv6

If you've associated an IPv6 CIDR block with your VPC, you can associate an IPv6 CIDR block with an existing subnet in your VPC, or when you create a new subnet. Possible IPv6 netmask lengths are between /44 and /64 in increments of /4.

prefixHexadecimalBinary number
/562a05:d01c:f8f:91 ~0010101000000101:1101000000011100:0000111110001111:10010001 ~
/642a05:d01c:f8f:9100 ~ 2a05:d01c:f8f:91ff0010101000000101:1101000000011100:0000111110001111:1001000100000000 ~ 0010101000000101:1101000000011100:0000111110001111:1001000111111111
profile picture
EXPERT
Riku_Kobayashi
answered a month ago
profile picture
EXPERT
Oleksii Bebych
reviewed a month ago
EXPERT
Leo K
reviewed a month ago
  • Leo K EXPERT
    a month ago

    To put it in another way, the /56 for your VPC indicates the size of a range of numbers, like an area code in a telephone network. The subnets are sub-allocations within that area code and have to have a longer prefix (just like a phone number) to fit inside the area code (=the CIDR of the VPC). You could think of the IPv6 address as a phone number 128 digits long, and the CIDR of size /56 having a prefix of 56 digits. So, if you allocate subnets of size /64, many of them will fit inside the /56 "area code" of your CIDR. If you assign the whole /56 to a subnet, all numbers would be used up.

  • em
    a month ago

    Thank you so much Riku and Leo!!!!!! Life savers

Relevant content