Adding a Secondary IPv4 CIDR Block to a VPC

Scenarios

Notes and Constraints

• You can allocate a subnet from either a primary or a secondary CIDR block of a VPC. A subnet cannot use both the primary and the secondary CIDR blocks.

  Subnets in the same VPC can communicate with each other by default, even if some subnets are allocated from the primary CIDR block and some are from the secondary CIDR block of a VPC.

• If a subnet in a secondary CIDR block of your VPC is the same as or overlaps with the destination of an existing route in the VPC route table, the existing route does not take effect.

  If you create a subnet in a secondary CIDR block of your VPC, a route (the destination is the subnet CIDR block and the next hop is Local) is automatically added to your VPC route table. This route allows communications within the VPC and has a higher priority than any other routes in the VPC route table. For example, if a VPC route table has a route with the VPC peering connection as the next hop and 100.20.0.0/24 as the destination, and a route for the subnet in the secondary CIDR block has a destination of 100.20.0.0/16, 100.20.0.0/16 and 100.20.0.0/24 overlaps and traffic will be forwarded through the route of the subnet.

• Table 1 provides you with IP address ranges that cannot be used as secondary IPv4 CIDR blocks. For example, the CIDR block 192.168.0.0/16 has IP addresses from 192.168.0.0 to 192.168.255.255, indicating that none of the IP addresses can be included in a secondary IPv4 CIDR block, for example, 192.168.0.0/16, 192.168.31.0/24, 192.168.100.0/24, and 192.168.255.255/32.

  Table 1 IP address ranges that cannot be used as secondary IPv4 CIDR blocks

  Type	CIDR Block	IP Address Range
  Reserved private CIDR blocks	172.31.0.0/16	172.31.0.0-172.31.255.255
  	192.168.0.0/16	192.168.0.0–192.168.255.255
  	In-use primary CIDR blocks	-
  Reserved system CIDR blocks	100.64.0.0/10	100.64.0.0–100.127.255.255
  	214.0.0.0/7	214.0.0.0–215.255.255.255
  	198.18.0.0/15	198.18.0.0–198.19.255.255
  	169.254.0.0/16	169.254.0.0–169.254.255.255
  Reserved public CIDR blocks	0.0.0.0/8	0.0.0.0-0.255.255.255
  	127.0.0.0/8	127.0.0.0–127.255.255.255
  	240.0.0.0/4	240.0.0.0–255.255.255.255

Procedure

1. Log in to the management console.
2. Click in the upper left corner and choose Network > Virtual Private Cloud.

   The Virtual Private Cloud page is displayed.

3. In the VPC list, locate target VPC and click Edit CIDR Block in the Operation column.

   The Edit CIDR Block dialog box is displayed.

4. Click Add Secondary IPv4 CIDR Block.

   For example, if the primary CIDR block of a VPC is 192.168.0.0/16, you can add secondary CIDR blocks 10.1.0.0/16, 10.2.0.0/16, 172.16.0.0/16, or 172.17.0.0/16 as a secondary CIDR block.

5. Enter a secondary CIDR block and click OK.