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    Elastic Compute Service:Overview

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    Elastic Compute Service:Overview

    Last Updated:Dec 13, 2024

    A prefix list is a set of one or more network prefixes (CIDR blocks). You can reference prefix lists to configure network rules for other resources. You can add frequently used CIDR blocks to prefix lists to eliminate the need to repeatedly add rules for CIDR blocks when you configure network rules. This improves O&M efficiency. Prefix lists can be referenced in security group rules.

    Terms

    Term

    Description

    maximum number of entries

    The maximum number of CIDR blocks in a prefix list. Each entry consists of a CIDR block and the description for the CIDR block.

    address family

    The address family of entries in a prefix list. Prefix lists support the IPv4 or IPv6 address family. Entries in a prefix list must belong to the same address family.

    CIDR block

    The CIDR block is an addressing method used over the Internet, which is more efficient in IP address allocation compared to traditional Classes A, B, and C addressing models. CIDR blocks uses the slash notation in the following format: <IP address>/<Number of network ID bits.

    • Example 1: Convert a CIDR block to an IP address range 10.0.0.0/8 can be converted into a 32-bit binary address of 00001010.00000000.00000000.00000000. In the IP address, /8 indicates 8 bits of the network ID, meaning that the left-most 8 bits in the 32-bit binary address are fixed to the network ID. The corresponding IP address range is from 00001010.00000000.00000000.00000000 to 00001010.11111111.11111111.11111111. The subnet mask of 10.0.0.0/8 is 255.0.0.0 in decimal notation, which indicates the IP address range from 10.0.0.0 to 10.255.255.255.

    • Example 2: Convert an IP address range to the CIDR block format

      In an IP address range from 192.168.0.0 to 192.168.31.255, the right-most two octets in the IP addresses are converted into 00000000.00000000 to 00011111.11111111 in the binary format. Accordingly, the left-most 19 bits (= 8 × 2 + 3) are fixed to the network ID. This way, the IP address segment can be converted to 192.168.0.0/19 in the CIDR block format.

    associated resource

    A resource that references a prefix list.

    Limits

    Item

    Limit

    Adjustable

    Maximum number of prefix lists per region in an account

    100

    N/A

    Maximum number of entries in a single prefix list

    200

    N/A

    Maximum number of resources that can be associated with a prefix list

    1,000

    N/A

    References