CIDR blocks and IP addressing in AWS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does CIDR stand for and what is its purpose?

CIDR stands for Classless Inter-Domain Routing. It helps allocate IP addresses more efficiently by allowing flexible block sizes instead of fixed classes.

beginner

Explain the format of a CIDR block, for example, 192.168.1.0/24.

A CIDR block has an IP address followed by a slash and a number. The number (/24) shows how many bits are fixed for the network part. Here, /24 means the first 24 bits are network, leaving 8 bits for hosts.

intermediate

How many IP addresses are available in a /28 CIDR block?

A /28 block has 32 - 28 = 4 bits for hosts. That means 2^4 = 16 IP addresses total. Usually, 2 are reserved (network and broadcast), so 14 usable addresses.

beginner

Why is CIDR important in AWS VPC networking?

CIDR lets you define the size of your VPC and subnets precisely. This helps avoid wasting IP addresses and allows better control over network design.

beginner

What is the difference between public and private IP addresses in CIDR blocks?

Public IPs are reachable on the internet, while private IPs are used inside private networks. Private IP ranges are defined by specific CIDR blocks like 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.

What does the '/16' mean in the CIDR block 10.0.0.0/16?

A It is a subnet mask of 255.255.255.0

B The last 16 bits are the network part

C The first 16 bits are the network part

D There are 16 IP addresses total

In CIDR, the number after the slash shows how many bits are fixed for the network. /16 means the first 16 bits are network.

How many usable IP addresses are in a /30 subnet?

A 6

B 4

C 8

D 2

A /30 subnet has 2 bits for hosts (32-30=2), so 2^2=4 total addresses. 2 are reserved, leaving 2 usable IPs.

Which CIDR block is a private IP range?

A 192.168.1.0/24

B 8.8.8.0/24

C 172.32.0.0/12

D 100.64.0.0/10

192.168.1.0/24 is within the private IP range 192.168.0.0/16. 8.8.8.0/24 is public, 172.32.0.0/12 is outside private range, 100.64.0.0/10 is shared address space.

Why should you avoid overlapping CIDR blocks in AWS VPCs?

A It causes IP address conflicts and routing issues

B It increases network speed

C It reduces security

D It automatically merges the networks

Overlapping CIDR blocks cause IP conflicts and routing problems, making communication between networks unreliable.

What is the subnet mask equivalent of /24?

A 255.255.0.0

B 255.255.255.0

C 255.0.0.0

D 255.255.255.255

/24 means 24 bits for network, which corresponds to 255.255.255.0 subnet mask.

Describe what a CIDR block is and how it helps in IP addressing.

Think about how IP addresses are grouped and counted.

Explain why choosing the right CIDR block size is important when creating an AWS VPC.

Consider how many devices you want to connect and how IPs are assigned.