VPC CIDR Calculations Flashcards
What is the largest CIDR block range in AWS?
10.0.0.0/16
For the certified SysOps certification. which CIDR value will you be tested on?
IP address Count
There are four values that a CIDR block range provides, what are they?
Netmask
First IP
Last IP
IP address Count
How many IP addresses are included in 10.0.0.0/16
65,536
In AWS, what is the smallest number you can put after the forward slash in a CIDR block? ex. 10.0.0.0/x
16
In AWS, what is the largest number you can put after the forward slash in a VPC CIDR block? ex. 10.0.0.0/x
- This will provide 16 IP addresses
After every iteration of one after the forward slash, available IP addresses are removed by how many?
Available IP addresses are reduced by one half.
/26 = 64 IP addresses. /27 = 32, /28 = 16, and so on.
In calcuating how many IP addresses your VPC needs, remember that AWS reserves a number of IP addresses that you will not have access to. How many IP addresses does AWS reserve?
5
The AWS SysOps test will be primarily concerned aout your ability to clacluate which CIDR block ranges?
/24 through /28
How many IP addresses in /24
256 - 5 reserved
How many IP addresses in /25
128 - 5 reserved
How many IP addresses in /26
64 - 5 reserved
How many IP addresses in /27
32 - 5 reserved
How many IP addresses in /28
16 - 5 reserved
You need a small VPC with only a few IP addresses and decide that a /29 will be sufficient. You get an error when you attempt to create it. What’s the likely problem?
AWS does not allow VPCs created with a CIDR range smaller than /28
You need to create a VPC with 50 IP addresses available, what is the most appropriate CIDR block range?
/26. This provides 64 IP addresses, minus 5 reserved, which equals 59 IP addresses.
You need to create a VPC with 200 IP addresses available. Which CIDR range is most appropriate?
/24. This provides 256 IP addresses, minus 5 reserved, which equals 251 IP addresses available.
The SysOps exam asks you to calculate a CIDR block range that will provide 60 available IP addresses. You answer /26 since it provides 64 IP addresses. You later find out your answer was wrong. Why might this be?
You forgot to consider the reserved IP addresses. Since you need 60 available IP addresses, a /26 will only make 59 IP addresses available after the 5 reserved IP addresses. Therefor the approriate answer would be /25.