CIDR ranges examples

When you create a subnet, you specify the CIDR block for the subnet, which is a subset of the VPC CIDR block. For IPv4, you cannot create a subnet with a CIDR block larger than a /16. For IPv6, the subnet size is fixed to be a /64 CIDR block.

AWS VPC CIDR block ranges:

IPv4

• Minimum size: /28 (16 IP addresses)

• Maximum size: /16 (65,536 IP addresses)

• Must be from private IPv4 address ranges:

  • 10.0.0.0/8

  • 172.16.0.0/12

  • 192.168.0.0/16

IPv6

• Fixed size: /56 for VPC

• Fixed size: /64 for subnets

CIDR	Netmask	2^ Calculation	Available IPs	Common Use Cases
/32	255.255.255.255	2^(32-32) = 2^0	1	Single host address (e.g., firewall rules)
/31	255.255.255.254	2^(32-31) = 2^1	2	Point-to-point links between routers
/30	255.255.255.252	2^(32-30) = 2^2	4	Router-to-router WAN links
/29	255.255.255.248	2^(32-29) = 2^3	8	Very small office networks, IoT segments
/28	255.255.255.240	2^(32-28) = 2^4	16	Small branch offices, home labs
/27	255.255.255.224	2^(32-27) = 2^5	32	Small department networks
/26	255.255.255.192	2^(32-26) = 2^6	64	Medium department networks
/25	255.255.255.128	2^(32-25) = 2^7	128	Large department networks
/24	255.255.255.0	2^(32-24) = 2^8	256	Standard enterprise subnet, AWS default subnet
/23	255.255.254.0	2^(32-23) = 2^9	512	Large subnet spanning departments
/22	255.255.252.0	2^(32-22) = 2^10	1,024	Campus networks, data center segments
/21	255.255.248.0	2^(32-21) = 2^11	2,048	Small ISP allocations
/20	255.255.240.0	2^(32-20) = 2^12	4,096	Medium ISP allocations
/19	255.255.224.0	2^(32-19) = 2^13	8,192	Large ISP allocations
/18	255.255.192.0	2^(32-18) = 2^14	16,384	Regional ISP networks
/17	255.255.128.0	2^(32-17) = 2^15	32,768	Large regional networks
/16	255.255.0.0	2^(32-16) = 2^16	65,536	AWS VPC default, organization networks
/8	255.0.0.0	2^(32-8) = 2^24	16,777,216	Largest orgs, legacy Class A
/0	0.0.0.0	2^(32-0) = 2^32	4,294,967,296	Entire IPv4 address space

Examples

1. What's the number of available IP addresses in:

• /24 = 256 IPs (254 usable)

• /28 = 16 IPs (14 usable)

• /27 = 32 IPs (30 usable)

• /16 = 65,536 IPs (65,534 usable)

1. Given VPC 10.0.0.0/16:

• What's the largest possible subnet? (/16)

• What's a valid subnet CIDR? (10.0.1.0/24)

• Is 11.0.0.0/24 valid? (No, outside VPC range)

1. For 192.168.1.0/24:

• First usable IP? (192.168.1.1)

• Last usable IP? (192.168.1.254)

• Broadcast address? (192.168.1.255)

1. Can these subnets overlap? 10.0.0.0/24 and 10.0.0.128/25 (Yes) 10.0.0.0/24 and 10.0.1.0/24 (No)

2. Split 10.0.0.0/24 into four equal subnets:

• 10.0.0.0/26

• 10.0.0.64/26

• 10.0.0.128/26

• 10.0.0.192/26