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Getting Started with Networking: Understand  AWS VPC Subnet IP Address Reservation

By: Author Alex Lim

Posted on

Categories Networking

Learn about the number of IP addresses AWS reserves when creating a subnet in a VPC. Discover the purpose behind these reserved IP addresses and their impact on your network design.

Table of Contents

Question

How many IP addresses does AWS reserve when you create a subnet in a VPC?

A. Four IP addresses
B. Two IP addresses
C. One IP address
D. Five IP addresses

Answer

D. Five IP addresses

Explanation

AWS reserves five IP addresses in every subnet: the first four IPs and the last IP.

When you create a subnet in an Amazon Virtual Private Cloud (VPC), AWS reserves five IP addresses within each subnet’s CIDR block. These reserved IP addresses serve specific purposes and are not available for use by instances launched in the subnet.

The five reserved IP addresses in each subnet are:

  1. Network Address: The first IP address in the subnet’s CIDR block. It represents the subnet itself and is used for identification purposes.
  2. VPC Router: The second IP address is reserved for the VPC router, which enables communication between subnets and allows instances to access the internet through an internet gateway.
  3. DNS Server: The third IP address is used by the Amazon DNS server, which provides domain name resolution for instances within the VPC.
  4. Future Use: The fourth IP address is reserved by AWS for future use and is currently not utilized.
  5. Network Broadcast Address: The last IP address in the subnet’s CIDR block is the network broadcast address. It is not assigned to any instance and is used for broadcast communication within the subnet.

It’s important to consider these reserved IP addresses when designing your VPC subnets and calculating the number of available IP addresses for your instances. By understanding the purpose behind each reserved IP address, you can effectively plan your network architecture and ensure efficient utilization of the available IP address space within your subnets.

Therefore, the correct answer to the question “How many IP addresses does AWS reserve when you create a subnet in a VPC?” is option D: Five IP addresses.

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