Module 2: The Network Layer

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In this post, I provide complete, accurate, and detailed explanations for the answers to Module 2: The Network Layer of Course 2: The Bits and Bytes of Computer Networking – Google IT Support Professional Certificate

Whether you’re preparing for quizzes or brushing up on your knowledge, these insights will help you master the concepts effectively. Let’s dive into the correct answers and detailed explanations for each question!

The Network Layer

Practice Quiz

1. Please select all of the valid IP addresses. Check all that apply.

  • 8.8.8.8 ✅
  • 123.456.123.456
  • 192.168.1.1 ✅
  • 257.70.312.49

Explanation:
A valid IPv4 address consists of four octets, each ranging from 0 to 255.

2. How many IP addresses does a class C network have?

  • 16,777,216 addresses
  • 256 addresses ✅
  • 65,536 addresses
  • 1 address

Explanation:
A Class C network has a default subnet mask of 255.255.255.0, which allows 256 total addresses (0-255), but only 254 are usable (excluding network & broadcast addresses).

3. Which IP address class has the first two octets reserved for the network ID and the last two octets for the host ID?

  • Class B ✅
  • Class A
  • Class E
  • Class C addresses

Explanation:

  • Class A: First octet for network ID, remaining three for hosts.
  • Class B: First two octets for network ID, last two octets for hosts.
  • Class C: First three octets for network ID, last octet for hosts.
  • Class E: Reserved for future use, not for public allocation.

4. The ____ is used to indicate that the datagram is allowed to be fragmented or to indicate that the datagram has already been fragmented.

  • service type
  • field fragmentation offset field
  • total length field
  • flag field ✅

Explanation:
The flag field in an IP header includes:

  • DF (Don’t Fragment) bit – Prevents fragmentation
  • MF (More Fragments) bit – Indicates more fragments exist

5. The IP address assigned to a device would depend on the _______ it is connected.

  • network ✅
  • ARP
  • MAC addresses
  • IP datagram

Explanation:
An IP address is assigned based on the network a device connects to. Each network has a specific subnet and address range.

Subnetting

Practice Quiz

6. What does CIDR stand for?

  • Classful Identification Routing
  • Class Interconnect Destination Routing
  • Classful Interlink-Destination Routing
  • Classless Inter-Domain Routing ✅

Explanation:
CIDR is a method for allocating IP addresses and routing that replaces the older classful addressing system. It allows for more efficient use of IP addresses by enabling variable-length subnet masking (VLSM).

7. Which of the following is a correct form of CIDR notation?

  • 192.168.1.0 + 255.255.255.0
  • 192.168.1.0/24 ✅
  • 192.168.1.0:24
  • 192.168.1.0\24

Explanation:
CIDR notation represents IP addresses and subnet masks in the format:
IP_address/prefix_length (e.g., 192.168.1.0/24 means a subnet mask of 255.255.255.0).

8. How many octets does a subnet mask have?

  • 4 ✅
  • 1
  • 3
  • 2

Explanation:
An IPv4 subnet mask consists of four octets (just like an IPv4 address). Example:

  • 255.255.255.0
  • 255.255.0.0
  • 255.255.255.252

Each octet represents 8 bits, making the subnet mask 32 bits in total.

9. Consider the following scenario:

Your IP address is 192.168.8.34, and your subnet mask is 255.255.0.0. What part of the subnet mask represents the subnet ID?

  • The last two octets of the subnet mask, 0.0
  • The first two octets of the subnet mask, 255.255 ✅
  • 192.168.0.254
  • 255.255.0.1

Explanation:
A /16 subnet mask (255.255.0.0) means:

  • The first two octets (192.168.) represent the network ID.
  • The last two octets (8.34) represent the host ID.

This means all IPs starting with 192.168.x.x belong to the same subnet.

10. How many possible host IDs do you always lose per network?

  • 2 ✅
  • 12
  • 4
  • 8

Explanation:
In every subnet, two addresses are reserved and cannot be assigned to hosts:
Network Address → Identifies the network itself (e.g., 192.168.1.0/24)
Broadcast Address → Used to send data to all hosts in the subnet (e.g., 192.168.1.255/24)

So, always subtract 2 from the total available IPs in a subnet.

Routing

Practice Quiz

11. Which of the following are types of Interior Gateway Protocols? (Select all that apply)

  • Link State Routing Protocols ✅
  • Distance-Vector Protocols. ✅
  • Hypertext Transfer Protocol
  • Transmission Control Protocol

Explanation:
Interior Gateway Protocols (IGPs) are used within a single autonomous system (AS) to route data efficiently. The two main types are:
Distance-Vector Protocols → Example: RIP (Routing Information Protocol)
Link State Routing Protocols → Example: OSPF (Open Shortest Path First)

12. Who is permitted to use non-routable address space?

  • It’s for testing purposes only
  • The IANA
  • The IETF
  • Anyone ✅

Explanation:
Private IP addresses (non-routable) are used freely by anyone within a local network. These addresses cannot be routed over the public internet and require NAT (Network Address Translation) for external access.

13. A typical routing table may contain which of the following? Check all that apply.

  • Total Hops ✅
  • TTL
  • Destination Network ✅
  • Destination address

Explanation:
A routing table contains:
Destination Network → The network that packets are being sent to.
Total Hops → The number of routers a packet must pass through.

14. What is the difference between Interior Gateway Protocols and Exterior Gateway Protocols?

  • Interior gateway protocols share information with public networks. Exterior gateway protocols are used for private networks.
  • Interior Gateway Protocols share information within a single autonomous system. Exterior Gateway Protocols are used for the exchange of information between multiple autonomous systems. ✅
  • Interior gateway protocols share information with multiple autonomous systems. Exterior Gateway Protocols are used for the exchange of information between independent autonomous systems.
  • There is no difference between them.

Explanation:
Interior Gateway Protocols (IGPs) → Used inside an Autonomous System (AS) (e.g., OSPF, RIP, EIGRP).
Exterior Gateway Protocols (EGPs) → Used between different ASes (e.g., BGP – Border Gateway Protocol).

15. Which of the following are non-routable IP addresses? (Select all that apply)

  • 172.16.0.0/12 ✅
  • 192.168.0.0/16. ✅
  • 9.0.0.0
  • 10.0.0.0/8 ✅

Explanation:
Non-routable (Private) IP Ranges:

  • 10.0.0.0 – 10.255.255.255 (/8 range)
  • 172.16.0.0 – 172.31.255.255 (/12 range)
  • 192.168.0.0 – 192.168.255.255 (/16 range)

Module 2 challenge: The Network Layer

Graded Quiz

16. No matter where a device is used, what address will stay the same regardless of the network it is attached to?

  • Physical, or MAC, ✅ address
  • Dynamic IP address
  • Static IP address
  • EIN serial

Explanation:
A MAC (Media Access Control) address is a unique hardware address assigned to a device’s network interface card (NIC). It remains constant across networks, unlike IP addresses that may change.

17. How many bits long is an IP address?

  • 8 bits
  • 256 bits
  • 64 bits
  • 32 bits ✅

Explanation:
IPv4 addresses are 32-bit binary numbers, typically represented as four decimal octets (e.g., 192.168.1.1).
IPv6, however, is 128 bits long.

18. In a datagram, what does the flag field indicate?

  • If a datagram is allowed to or has already been fragmented ✅
  • If a datagram has degraded and become unusable
  • If a datagram is high or low priority
  • If a datagram should be scanned for viruses

Explanation:
The flag field in an IP datagram controls fragmentation:

  • DF (Don’t Fragment) → Prevents fragmentation.
  • MF (More Fragments) → Indicates more fragments follow.

19. Which IP address is Class C?

  • 192.37.48.98 ✅
  • 224.24.45.69
  • 128.42.39.72
  • 172.38.14.72

Explanation:
IP Class Ranges:

  • Class A: 1.0.0.0 – 126.255.255.255
  • Class B: 128.0.0.0 – 191.255.255.255
  • Class C: 192.0.0.0 – 223.255.255.255
  • Class D: 224.0.0.0 – 239.255.255.255 (Multicast)

20. Why do entries in a local Address Resolution Protocol (ARP) table expire after a short amount of time?

  • It only needs to be used one time
  • To account for network changes ✅
  • To keep space in the table
  • It will use too much memory

Explanation:
ARP tables store mappings between IP addresses and MAC addresses. Since devices can change networks (e.g., moving from wired to Wi-Fi), ARP entries expire to ensure accuracy.

21. What does CIDR stand for?

  • Classic Intra-Demo Ratings
  • Classless Inter-Domain Routing ✅
  • Costless Inter-Data Routing
  • Clear Inter-Developer Relations

Explanation:
CIDR (Classless Inter-Domain Routing) replaces the old class-based IP system and allows for efficient IP address allocation using subnet masks (e.g., 192.168.1.0/24).

22. A router is performing basic routing functions. What is the third step in the transmission of a packet?

  • The router forwards the packet.
  • The router looks up the destination network in its routing table. ✅
  • A router receives a packet of data.
  • The router examines the destination IP.

Explanation:
The correct sequence in routing:
Router receives a packet.
Router examines the destination IP.
Router looks up the destination network in its routing table.
Router forwards the packet to the next hop.

23. Link state routing protocols are newer and they require…

  • Modern encryption standards
  • Manual updating
  • Less memory and less processing power
  • More memory and more processing power ✅

Explanation:
Link-state protocols (e.g., OSPF, IS-IS) build a complete map of the network using Dijkstra’s Algorithm. This requires more processing power and memory than distance-vector protocols like RIP.

24. What organization helps manage IP address allocation and autonomous system number allocation?

  • IANA ✅
  • INNA
  • ICANN
  • IBM

Explanation:
IANA (Internet Assigned Numbers Authority) manages:

  • IP address blocks
  • Autonomous System Numbers (ASNs)
  • Root name servers

25. Ranges of IP addresses anyone can use for their internal networks are known as ______.

  • MAC address
  • IP address
  • Non-routable address space ✅
  • Subnet mask

Explanation:
Non-routable (Private) IP Ranges:

  • 10.0.0.0 – 10.255.255.255 (/8 range)
  • 172.16.0.0 – 172.31.255.255 (/12 range)
  • 192.168.0.0 – 192.168.255.255 (/16 range)

26. In a Class A address, the possible values for the first octet are:

  • 128 through 199
  • 0 through 255
  • 0 through 127 ✅
  • 255 through 420

Explanation:

  • Class A addresses range from 0.0.0.0 to 127.255.255.255.
  • The first octet (8 bits) determines the network portion, while the remaining three octets (24 bits) are for host addresses.
  • The first bit in a Class A address is always 0, which is why its range is 0 to 127.

27. What does ARP stand for?

  • Augmented Resolution Picture
  • Aggressive Resource Protection
  • Anonymous Resource Protocol
  • Address Resolution Protocol ✅

Explanation:

  • ARP (Address Resolution Protocol) is used to map an IP address to a MAC address in a local network.
  • When a device wants to communicate with another device, it uses ARP to find the corresponding MAC address before sending data.

28. How many bits long is an Autonomous System Number (ASN)?

  • 32 ✅
  • 8
  • 16
  • 64

Explanation:

  • An Autonomous System Number (ASN) is a unique identifier assigned to networks that route data using BGP (Border Gateway Protocol).
  • Originally, ASNs were 16-bit numbers, but due to the growth of the internet, 32-bit ASNs were introduced.

29. Which of the following is a valid IP address?

  • A8:97:4F:6D:H6:0F
  • 126.396.659.100
  • 598.36.789
  • 128.42.64.78 ✅

Explanation:

  • A valid IPv4 address consists of four octets (0-255) separated by dots.
  • The invalid options:
    • A8:97:4F:6D:H6:0F (This is a MAC address format, not an IP address).
    • 126.396.659.100 (Each octet must be between 0 and 255; 396 and 659 are invalid).
    • 598.36.789 (Not in the correct four-octet format, and numbers exceed the valid range).

30. There are ____ octets in an IP address.

  • 2
  • 8
  • 16
  • 4 ✅

Explanation:

  • An IPv4 address consists of four octets, each containing 8 bits (total 32 bits).
  • Example: 192.168.1.1 → Four sections (octets), each between 0-255.

31. Which IP address is Class C?

  • 128.42.39.72
  • 172.38.14.72
  • 192.37.48.98 ✅
  • 224.24.45.69

Explanation:

  • Class C addresses range from 192.0.0.0 to 223.255.255.255.
  • Breakdown of options:
    • 128.42.39.72 → Class B (128-191)
    • 172.38.14.72 → Class B (128-191)
    • 192.37.48.98 → ✅ Class C (192-223)
    • 224.24.45.69 → Class D (Multicast) (224-239)

32. Why do entries in a local Address Resolution Protocol (ARP) table expire after a short amount of time?

  • To account for network changes ✅
  • It only needs to be used one time
  • To keep space in the table
  • It will use too much memory

Explanation:

  • The ARP table stores IP-to-MAC address mappings but entries expire quickly (typically within minutes).
  • This ensures devices that change networks or MAC addresses do not cause communication issues.
  • If an entry expires, a new ARP request is sent to update the table.

33. What is one main reason CIDR is helpful in modern networking?

  • CIDR simplifies MAC address assignment
  • CIDR lowers the amount of power needed to run a server
  • CIDR allows for more arbitrary network sizes ✅
  • CIDR increases network security with no additional resources

Explanation:

  • CIDR (Classless Inter-Domain Routing) eliminates fixed class-based networks (Class A, B, C).
  • It allows flexible subnetting by using variable-length subnet masks (VLSM) (e.g., /24, /30, /28).
  • This optimizes IP address allocation, preventing wastage.

34. What does RFC stand for?

  • Request For Comments ✅
  • Redundant Frame Controller
  • Routing Frequency Control
  • Realtime Frame Check

Explanation:

  • RFCs (Request for Comments) are official technical documents published by the Internet Engineering Task Force (IETF).
  • They define internet standards, protocols, and best practices.

35. Generally, what are Class E IP addresses used for?

  • CIDR
  • Broadcasting
  • Multicasting
  • Testing ✅

Explanation:

  • Class E IP addresses (240.0.0.0 – 255.255.255.255) are reserved for experimental and research purposes.
  • They are not used for normal networking.

Related contents:

Module 1: Introduction to Networking
Module 3: The Transport and Application Layers
Module 4: Networking Services
Module 5: Connecting to the Internet
Module 6: Troubleshooting and the Future of Networking

You might also like:

Course 1: Technical Support Fundamentals
Course 3: Operating Systems and You: Becoming a Power User
Course 4: System Administration and IT Infrastructure Services
Course 5: IT Security: Defense against the digital dark arts

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