IP packet structure in Computer Networks - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to name the first field of an IP packet header.

Computer Networks

The first field in an IP packet header is called the [1] field.

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AChecksum

BVersion

CPayload

DSource Address

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the Version field with the Checksum or Source Address fields.

The first field in an IP packet header is the Version field, which indicates the IP protocol version.

2fill in blank

medium

Complete the code to identify the field that specifies the total length of the IP packet.

Computer Networks

The field that specifies the total length of the IP packet is called the [1] field.

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AIdentification

BTime to Live

CFlags

DTotal Length

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up Total Length with Identification or Flags fields.

The Total Length field specifies the entire size of the IP packet, including header and data.

3fill in blank

hard

Fix the error in naming the field that controls packet fragmentation.

Computer Networks

The [1] field controls fragmentation and reassembly of IP packets.

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AFlags

BHeader Checksum

CProtocol

DTime to Live

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing Flags with Time to Live or Protocol fields.

The Flags field contains bits that control fragmentation, such as 'Don't Fragment' and 'More Fragments'.

4fill in blank

hard

Fill both blanks to complete the description of the source and destination addresses in an IP packet.

Computer Networks

The [1] field contains the sender's IP address, and the [2] field contains the receiver's IP address.

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ASource Address

BDestination Address

CIdentification

DHeader Length

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up Source and Destination Address fields.

The Source Address field holds the sender's IP, and the Destination Address field holds the receiver's IP.

5fill in blank

hard

Fill all three blanks to complete the dictionary comprehension that maps IP header fields to their typical bit sizes, filtering only fields larger than 8 bits.

Computer Networks

ip_header_bits = { [1]: [2] for [3], [4] in fields.items() if [4] > 8 }

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Afield

Bsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same variable name for both key and value.

Not matching the variable names in the for loop with those in the comprehension.

This comprehension creates a dictionary of IP header fields and their bit sizes, including only those fields larger than 8 bits. The variables field and size represent the key and value from the fields dictionary.

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