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CIDR Notation

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Classless Inter-Domain Routing (CIDR) is a notation used to depict IP addresses and their associated routing prefix. It allows for more efficient use of IP address space and improved route aggregation, which makes routing more efficient and secure. CIDR notation consists of an IP address followed by a slash and the prefix length (the number of leading bits in the subnet mask). This notation replaces the traditional method of using IP classes and subnet masks, making it easier to allocate and manage IP addresses. CIDR contributes to reducing the load on routers and preventing the exhaustion of available IP addresses.

Guide on CIDR Notation

CIDR Notation stands for Classless Inter-Domain Routing, an IP addressing scheme that effectively replaced the older system known as Classful IP Addressing.

Importance:
CIDR allows for more efficient use of IP addresses and address space. It enables one IP address to designate many unique IP addresses. It is integral to IP routing, as it consolidates route information.

What is it:
CIDR is basically the combination of an IP address and its associated network prefix size. This is normally expressed as 'IP address slash prefix size', like 192.0.2.0/24. Here, '192.0.2.0' is the IP address and '24' denotes the size of the network prefix.

How it works:
With CIDR, an IP network is represented by a prefix. This is an IP address followed by a slash and the count of consecutive '1' bits in the network mask. For example, 192.0.2.0/24 is a CIDR block. The /24 after the IP address indicates that 24 of the 32 bits are the network part of the address, leaving the remaining bits (8 in this case) for specific host addresses.

Exam Tips: Answering Questions on CIDR Notation
Tip#1: Key terminology: Familiarize yourself with terms such as 'subnet mask', 'network address', 'broadcast address' and 'host addresses'.
Tip#2: CIDR Practice: Practice IP address calculation for CIDR notations. Websites like subnetting.net provide a practical ground for CIDR notation based calculations.
Tip#3: Subnetting: Understand the concept of subnetting and how CIDR allows us to divide IP addresses into subnets.
Tip#4: Relationship between subnet mask and prefix: To deduce the subnet from CIDR notation, subtract the prefix size from 32. That means /24 corresponds to a subnet mask of 255.255.255.0, as each octet in the mask is 8 bits. 32-24=8 bits left for host addressing.
Tip#5: Real-world scenarios: Master the application of CIDR in real world scenarios, solving problems based on it.

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CompTIA Network+ - IP Addressing Example Questions

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Question 1

A network administrator needs to configure a subnet containing 8 IP addresses for a printer pool. What would be the best CIDR notation for this subnet?

Correct Answer: /29

The best CIDR notation for a subnet containing 8 IP addresses would be /29. This is because a /29 subnet provides exactly 8 IP addresses (even though 2 are reserved for network address and broadcast address, the question assumes we require 8 addresses in total, not 8 usable ones). A /28, /27 or /26 subnet provides more addresses (16, 32, and 64 respectively), which is unnecessary when you only need 8, hence wasting the IP addresses. Therefore, /29 is the most efficient and best choice.

Question 2

An IT team is tasked to implement a subnet with 50 IP addresses for a new server cluster within a company's network. What CIDR notation should they use to accommodate the right amount of IP addresses?

Correct Answer: /26

CIDR notation /26 specifies a subnet that can accommodate up to 64 IP addresses, which is more than sufficient for the requirement of 50 addresses. Taking into account the network address and the broadcast address which cannot be assigned to any device, we are left with 62 usable addresses. The other choices provide either less than or excess number of addresses. For instance, /25 provides 128 addresses, which is excessive; /28 provides only 16 addresses, which is too few; and /29 provides a mere 8 addresses, still insufficient. Hence /26 is the best-suited option.

Question 3

A network administrator is tasked with dividing an IP address range 192.168.0.0/24 into smaller subnets for different departments of a company. The first subnet of 40 IP addresses is for Finance. What should be the CIDR notation used for this subnet?

Correct Answer: /26

An IP address in CIDR notation with /24 means that the first 24 bits are used for the network, leaving 8 bits for hosts. The network can accommodate 256 addresses (calculated as 2 to the power of 8). As the requirement is for a subnet of 40 IP addresses, a /26 CIDR notation will be used. This uses the first 26 bits for the network, leaving 6 bits for hosts - providing 64 addresses (calculated as 2 to the power of 6), which is more than enough for the 40 required by the Finance department. The subnet size closest to the requirement yet larger should be chosen. A /25 would provide 128 addresses, a waste as the requirement is for only 40 addresses. A /24 would provide the entire 256 addresses, an even greater waste. On the other hand, a /29 would only provide 8 addresses, which is insufficient for the requirement.

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