IP packet structure in Computer Networks - Practice Problems & Coding Challenges

Challenge - 5 Problems

📋 Factual

intermediate

Identify the correct size of the IP header without options

What is the size in bytes of a standard IPv4 header without any options included?

A 40 bytes

B 24 bytes

C 32 bytes

D 20 bytes

💡 Hint

Think about the minimum fixed length of the IPv4 header fields.

The standard IPv4 header without options is always 20 bytes long. Options, if present, increase this size.

🧠 Conceptual

intermediate

Purpose of the Time To Live (TTL) field in an IP packet

What is the main purpose of the Time To Live (TTL) field in an IPv4 packet header?

A To indicate the packet's priority level for routing

B To specify the maximum number of hops a packet can take before being discarded

C To identify the source IP address of the packet

D To specify the total length of the IP packet

💡 Hint

Consider how networks prevent packets from circulating endlessly.

The TTL field limits how many routers (hops) a packet can pass through. Each router decreases TTL by one. When TTL reaches zero, the packet is discarded to avoid infinite loops.

🔍 Analysis

advanced

Determining the payload size from IP header fields

Given an IPv4 packet with a Total Length field value of 60 bytes and an Internet Header Length (IHL) of 5, what is the size of the payload (data) in bytes?

A 40 bytes

B 20 bytes

C 60 bytes

D 80 bytes

💡 Hint

Remember that IHL is measured in 32-bit words and Total Length includes header and data.

IHL of 5 means header length is 5 × 4 = 20 bytes. Total Length is 60 bytes, so payload = 60 - 20 = 40 bytes.

❓ Comparison

advanced

Difference between IPv4 and IPv6 header sizes

Which statement correctly compares the header sizes of IPv4 and IPv6 packets?

A IPv4 header is variable length, minimum 20 bytes; IPv6 header is fixed length of 40 bytes

B IPv4 header is fixed length of 40 bytes; IPv6 header is variable length, minimum 20 bytes

C Both IPv4 and IPv6 headers have fixed length of 20 bytes

D Both IPv4 and IPv6 headers have variable length depending on options

💡 Hint

Consider how options affect header length in IPv4 and IPv6.

IPv4 headers can vary in length due to optional fields, starting at 20 bytes minimum. IPv6 headers have a fixed length of 40 bytes, with extension headers handled separately.

❓ Reasoning

expert

Effect of incorrect checksum in IP header

What happens when an IPv4 packet arrives at a router with an incorrect header checksum?

A The router sends the packet back to the sender

B The router corrects the checksum and forwards the packet

C The router discards the packet and does not forward it

D The router forwards the packet without checking the checksum

💡 Hint

Think about how routers ensure data integrity before forwarding.

Routers verify the header checksum to detect errors. If the checksum is incorrect, the packet is considered corrupted and discarded to prevent forwarding bad data.

Practice

1. Which part of an IP packet contains the source and destination addresses?

easy

A. Payload

B. Trailer

C. Header

D. Checksum

Solution

1. Step 1: Understand IP packet parts

   An IP packet mainly has two parts: the header and the payload (data).

2. Step 2: Identify address location

   The header contains control information including source and destination IP addresses.

3. Final Answer:

   Header -> Option C

4. Quick Check:

   Addresses are in the header [OK]

Hint: Remember: addresses are always in the header [OK]

Common Mistakes:

• Confusing payload with header
• Thinking addresses are in the trailer
• Assuming checksum holds addresses

2. Which field in the IP packet header specifies the version of the IP protocol?

easy

A. Protocol

B. Time to Live (TTL)

C. Header Checksum

D. Version

Solution

1. Step 1: Recall IP header fields

   The IP header includes fields like Version, TTL, Protocol, and Checksum.

2. Step 2: Identify version field

   The Version field specifically indicates whether the packet uses IPv4 or IPv6.

3. Final Answer:

   Version -> Option D

4. Quick Check:

   Version field shows IP protocol version [OK]

Hint: Version field always shows IPv4 or IPv6 [OK]

Common Mistakes:

• Confusing TTL with version
• Thinking checksum holds version info
• Mixing Protocol field with version

3. Given an IP packet with TTL set to 1, what happens when it reaches a router?

medium

A. The packet is dropped and an error message is sent back

B. The packet is delivered to the destination immediately

C. The packet's TTL is reset to 64

D. The packet is forwarded to the next router

Solution

1. Step 1: Understand TTL purpose

   TTL (Time to Live) limits how many routers a packet can pass through to avoid endless loops.

2. Step 2: Analyze TTL=1 at router

   When TTL reaches 0 after decrement, the router drops the packet and sends an error (ICMP Time Exceeded) back.

3. Final Answer:

   The packet is dropped and an error message is sent back -> Option A

4. Quick Check:

   TTL 1 means drop and notify [OK]

Hint: TTL zero means drop packet and send error [OK]

Common Mistakes:

• Assuming packet is forwarded with TTL 1
• Thinking TTL resets automatically
• Believing packet reaches destination immediately

4. A network engineer notices packets are not reaching the destination. Which IP header field should they check first to debug routing issues?

medium

A. Time to Live (TTL)

B. Identification

C. Source Address

D. Header Length

Solution

1. Step 1: Identify fields related to routing

   TTL controls packet lifespan; if too low, packets get dropped before reaching destination.

2. Step 2: Check TTL for routing problems

   Low or zero TTL causes packets to be discarded, causing delivery failure.

3. Final Answer:

   Time to Live (TTL) -> Option A

4. Quick Check:

   TTL affects packet routing success [OK]

Hint: Check TTL first when packets fail to route [OK]

Common Mistakes:

• Checking Identification instead of TTL
• Assuming Source Address causes routing failure
• Ignoring TTL impact on packet delivery

5. You want to create a custom IP packet for testing that includes a header with a specific Protocol field value indicating TCP. Which Protocol number should you set in the IP header?

hard

A. 1

B. 6

C. 17

D. 255

Solution

1. Step 1: Recall common IP Protocol numbers

   Protocol field indicates the next layer protocol: 1=ICMP, 6=TCP, 17=UDP, 255=Reserved.

2. Step 2: Identify TCP protocol number

   TCP is assigned protocol number 6 in the IP header.

3. Final Answer:

   6 -> Option B

4. Quick Check:

   TCP protocol number is 6 [OK]

Hint: TCP protocol number is 6 in IP header [OK]

Common Mistakes:

• Confusing UDP (17) with TCP
• Using ICMP (1) instead of TCP
• Choosing reserved number 255