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    Elastic Compute Service:Overview

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    Elastic Compute Service:Overview

    Last Updated:May 15, 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

    CIDR is an addressing scheme for the Internet that allows for IP addresses to be assigned in a more efficient manner than the traditional scheme based on classes A, B, and C. CIDR notation is used to denote IP addresses and IP ranges. It consists of an IP address and a forward slash followed by a decimal number that denotes how many bits are in the network prefix.

    • Example 1: Convert a CIDR block into an IP address range

      For example, you can convert the 10.0.0.0/8 CIDR block into a 32-bit binary IP address of 00001010.00000000.00000000.00000000. In this CIDR block, /8 represents an 8-bit network ID. The first 8 bits of the 32-bit binary IP address are fixed, and the corresponding IP addresses are from 00001010.00000000.00000000.00000000 to 00001010.11111111.11111111.11111111. After you convert the preceding IP addresses into IP addresses in the decimal format, the 10.0.0.0/8 CIDR block indicates the IP addresses from 10.0.0.0 to 10.255.255.255 with a subnet mask of 255.0.0.0.

    • Example 2: Convert an IP address range into a CIDR block

      For example, you have a range of IP addresses from 192.168.0.0 to 192.168.31.255. You can convert the last two parts of the first and last IP addresses to binary numbers from 00000000.00000000 to 00011111.11111111. The first 19 (8 × 2 + 3) bits are fixed. After you convert the IP addresses to IP addresses in the CIDR format, the corresponding CIDR block is 192.168.0.0/19.

    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