Private IP Addressing

5 minutes 5 Questions

Private IP addressing is a scheme that reserves a range of IP addresses for use within private networks, without the need for these addresses to be globally unique or routable on the Internet. The Internet Assigned Numbers Authority (IANA) has defined three blocks of IP address ranges as private:10.0.0.0 to 10.255.255.255 (Class A), 172.16.0.0 to 172.31.255.255 (Class B), and 192.168.0.0 to 192.168.255.255 (Class C). These private IP addresses are used for communication within local networks and cannot be directly accessed from the public Internet. To connect private networks to the public Internet, Network Address Translation (NAT) is used, which translates private IP addresses into public IP addresses at the network's edge. Private IP addressing helps conserve global IPv4 address space, improves security by limiting external visibility of internal devices, and allows for communication within local networks without requiring a unique public IP address for every device.

CompTIA Network+: Private IP Addressing Guide and Exam Tips

What is Private IP Addressing?
Private IP Addressing is a method of assigning IP addresses in a private network. These addresses are used in local area networks (LANs) and are not routable on the internet.

Why is Private IP Addressing Important?
Private IP Addressing is important as it allows multiple devices within a network to communicate with each other. It also conserves the limited number of global IP addresses by using NAT (Network Address Translation) to map the private IPs to a public IP for internet connectivity.

How does Private IP Addressing work?
In a private network, each device is assigned a unique private IP address within the specific range. When a device communicates over the internet, its private IP is replaced with a public IP by the router using NAT.

Exam Tips: Answering Questions on Private IP Addressing
Understand the difference between private and public IP addressing.
Know the ranges of private IP addresses: 10.0.0.0 – 10.255.255.255, 172.16.0.0 – 172.31.255.255, 192.168.0.0 – 192.168.255.255.
Be aware of the role of NAT in private IP addressing.
Practice questions on IP addressing to be familiar with different scenarios and be able to determine valid and invalid IP addresses.

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Question 1

You are configuring a home network and notice that one of the devices has an IP address of 169.254.100.50. Which of the following issues may cause this IP address to be assigned?

DHCP server unavailable Device manually set to static IP Subnet mask mismatch IP address conflict

Correct Answer: DHCP server unavailable

An IP address of 169.254.x.x is known as an Automatic Private IP Addressing (APIPA) address. This gets assigned when a device is unable to reach a Dynamic Host Configuration Protocol (DHCP) server to get an IP address automatically. This can be due to the DHCP server being unavailable or due to a network connectivity issue. The other answers are incorrect because an IP address conflict would usually result in network disruption and not in a device getting an APIPA address. A subnet mask mismatch would lead to communication problems on the network but would not trigger the release of an APIPA address. Finally, if a device was manually set to a static IP, it would be unlikely to be in the APIPA range unless specifically set to that.

Question 2

A local company wants to ensure all their devices use private IP addresses. Which IP address range below would be a valid private IP address for the company to use in their network?

both 172.30.15.25 and 192.169.5.2 172.20.240.1 172.30.15.25 192.169.5.2

Correct Answer: both 172.30.15.25 and 192.169.5.2

The valid private IP address ranges are: 10.0.0.0 to 10.255.255.255 (10/8 prefix) 172.16.0.0 to 172.31.255.255 (172.16/12 prefix) 192.168.0.0 to 192.168.255.255 (192.168/16 prefix) Among the given options, the following are valid private IP addresses: 172.30.15.25 (belongs to the 172.16.0.0 to 172.31.255.255 range) 192.169.5.2 (belongs to the 192.168.0.0 to 192.168.255.255 range) The other two options are not valid private IP addresses: 172.20.240.1 (outside the valid 172.16.0.0 to 172.31.255.255 range) 224.1.3.7 (belongs to the multicast address range, which is not a private IP address range) Therefore, the correct answers are: 172.30.15.25 192.169.5.2

Question 3

A software development company is working on an application. The team needs to test the application in an environment with the private IP address 172.29.130.65. To provide a testing environment, the subnet mask should be set to allow a maximum of 1024 hosts. Which subnet mask should they use to meet the requirements?

255.255.252.0 255.255.255.128 255.255.254.0 255.255.255.0

Correct Answer: 255.255.252.0

The correct subnet mask is 255.255.252.0. The subnet mask is used to determine the number of hosts a network can have. In this case, to accommodate a maximum of 1024 hosts, a /22 subnet mask (255.255.252.0) is required, allowing for 1022 usable addresses (the network address and the broadcast address cannot be used). 255.255.254.0 (/23) and 255.255.255.0 (/24) only allows 510 and 254 hosts respectively which is less than required. 255.255.255.128 (/25) only allows 126 hosts which is far from the requirement.

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