VPC CIDR Calculations

Question 1

What is the largest CIDR block range in AWS?

Answer 1

10.0.0.0/16

Question 2

For the certified SysOps certification. which CIDR value will you be tested on?

Answer 2

IP address Count

Question 3

There are four values that a CIDR block range provides, what are they?

Answer 3

Netmask
First IP
Last IP
IP address Count

Question 4

How many IP addresses are included in 10.0.0.0/16

Answer 4

65,536

Question 5

In AWS, what is the smallest number you can put after the forward slash in a CIDR block? ex. 10.0.0.0/x

Answer 5

16

Question 6

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

Answer 6

28. This will provide 16 IP addresses

Question 7

After every iteration of one after the forward slash, available IP addresses are removed by how many?

Answer 7

Available IP addresses are reduced by one half.

/26 = 64 IP addresses. /27 = 32, /28 = 16, and so on.

Question 8

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?

Answer 8

5

Question 9

The AWS SysOps test will be primarily concerned aout your ability to clacluate which CIDR block ranges?

Answer 9

/24 through /28

Question 10

How many IP addresses in /24

Answer 10

256 - 5 reserved

Question 11

How many IP addresses in /25

Answer 11

128 - 5 reserved

Question 12

How many IP addresses in /26

Answer 12

64 - 5 reserved

Question 13

How many IP addresses in /27

Answer 13

32 - 5 reserved

Question 14

How many IP addresses in /28

Answer 14

16 - 5 reserved

Question 15

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?

Answer 15

AWS does not allow VPCs created with a CIDR range smaller than /28

Question 16

You need to create a VPC with 50 IP addresses available, what is the most appropriate CIDR block range?

Answer 16

/26. This provides 64 IP addresses, minus 5 reserved, which equals 59 IP addresses.

Question 17

You need to create a VPC with 200 IP addresses available. Which CIDR range is most appropriate?

Answer 17

/24. This provides 256 IP addresses, minus 5 reserved, which equals 251 IP addresses available.

Question 18

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?

Answer 18

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.