A user has created a VPC with CIDR 20.0.0.0/24. The user has created a public subnet with CIDR 20.0.0.0/25
and a private subnet with CIDR 20.0.0.128/25. The user has launched one instance each in the private and
public subnets. Which of the below mentioned options cannot be the correct IP address (private IP. assigned to
an instance in the public or private subnet?
A.
20.0.0.255
B.
20.0.0.132
C.
20.0.0.122
D.
20.0.0.55
Explanation:
When the user creates a subnet in VPC, he specifies the CIDR block for the subnet. In this case the user has
created a VPC with the CIDR block 20.0.0.0/24, which supports 256 IP addresses (20.0.0.0 to 20.0.0.255.. The
public subnet will have IP addresses between 20.0.0.0 – 20.0.0.127 and the private subnet will have IP
addresses between 20.0.0.128 – 20.0.0.255. AWS reserves the first four IP addresses and the last IP address ineach subnet’s CIDR block. These are not available for the user to use. Thus, the instance cannot have an IP
address of 20.0.0.255
A.
20.0.0.255
The first and the last IP address is reserved .128 .255
Plus Amazon reserves two more IPs per subnet totaling 4 reserved.
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Amazon reserves the first four (4) IP addresses and the last one (1) IP address of every subnet for IP networking purposes.
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Answer
A.
20.0.0.255
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a
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The answer is D.
The first four IP addresses and the last IP address in each subnet CIDR block are not available for you to use, and cannot be assigned to an instance. For example, in a subnet with CIDR block 10.0.0.0/24, the following five IP addresses are reserved:
10.0.0.0: Network address.
10.0.0.1: Reserved by AWS for the VPC router.
10.0.0.2: Reserved by AWS. The IP address of the DNS server is always the base of the VPC network range plus two; however, we also reserve the base of each subnet range plus two. For more information, see Amazon DNS Server.
10.0.0.3: Reserved by AWS for future use.
10.0.0.255: Network broadcast address. We do not support broadcast in a VPC, therefore we reserve this address.
http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html
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The answer is A.
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thanks nagaruna
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