IP packet structure in Computer Networks - Step-by-Step Execution

Concept Flow - IP packet structure

Start: IP Packet

↓

Header Fields

↓

Version

↓

Header Length

↓

Type of Service

↓

Total Length

↓

Identification

↓

Flags

↓

Fragment Offset

↓

Time to Live

↓

Protocol

↓

Header Checksum

↓

Source IP Address

↓

Destination IP Address

↓

Options (if any)

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Data (Payload)

The IP packet starts with a header containing fields like version, length, and addresses, followed by the data payload.

Execution Sample

Computer Networks

Version: 4
Header Length: 20 bytes
Total Length: 60 bytes
Source IP: 192.168.1.1
Destination IP: 8.8.8.8
Data: 40 bytes payload

This example shows an IPv4 packet with a 20-byte header and 40 bytes of data.

Analysis Table

Step	Field	Value	Description
1	Version	4	Indicates IPv4 protocol
2	Header Length	20 bytes	Length of the header, minimum 20 bytes
3	Type of Service	0	Default service, no special priority
4	Total Length	60 bytes	Total size of packet including header and data
5	Identification	54321	Unique ID for fragmentation
6	Flags	010	Don't fragment flag set
7	Fragment Offset	0	No fragmentation offset
8	Time to Live	64	Packet lifetime before discard
9	Protocol	6	TCP protocol indicated
10	Header Checksum	0x1c2b	Error-checking for header
11	Source IP Address	192.168.1.1	Sender's IP address
12	Destination IP Address	8.8.8.8	Receiver's IP address
13	Options	None	No options present
14	Data	40 bytes	Payload data carried by packet
15	End	-	Packet fully constructed and ready for transmission

💡 All fields processed; packet ready to send.

State Tracker

Field	Initial	After Step 1	After Step 5	After Step 10	Final
Version	-	4	4	4	4
Header Length	-	-	20 bytes	20 bytes	20 bytes
Total Length	-	-	-	60 bytes	60 bytes
Identification	-	-	54321	54321	54321
Flags	-	-	010	010	010
Fragment Offset	-	-	0	0	0
Time to Live	-	-	-	64	64
Protocol	-	-	-	6	6
Header Checksum	-	-	-	0x1c2b	0x1c2b
Source IP Address	-	-	-	192.168.1.1	192.168.1.1
Destination IP Address	-	-	-	8.8.8.8	8.8.8.8
Options	-	-	-	None	None
Data	-	-	-	-	40 bytes

Key Insights - 3 Insights

Why is the Header Length important and how does it affect the packet?

Header Length tells how big the header is. It is needed to know where the data starts. In the execution_table at Step 2, the header length is 20 bytes, so data starts after that.

What does the Flags field '010' mean in the packet?

Flags '010' means the 'Don't Fragment' bit is set. This prevents the packet from being split. See Step 6 in the execution_table for this value.

How does the Total Length relate to Header Length and Data size?

Total Length is the sum of Header Length and Data size. In the table, Total Length is 60 bytes (Step 4), Header Length is 20 bytes (Step 2), so Data is 40 bytes (Step 14).

Visual Quiz - 3 Questions

Look at the execution_table at Step 4. What does the Total Length field represent?

A The size of the header only

B The combined size of header and data

C The size of the data only

D The size of the options field

💡 Hint

Check Step 2 (Header Length) and Step 14 (Data) to see how Total Length sums them.

At which step in the execution_table is the Source IP Address set?

A Step 11

B Step 9

C Step 13

D Step 15

💡 Hint

Look for the row labeled 'Source IP Address' in the table.

If the Flags field was '000' instead of '010', what would change?

A Packet would have a longer header

B Packet would be discarded immediately

C Packet would be allowed to fragment

D Packet would have no data

💡 Hint

Refer to the explanation in key_moments about Flags at Step 6.

Concept Snapshot

IP Packet Structure:
- Starts with a header (min 20 bytes)
- Header includes Version, Length, Flags, Addresses
- Total Length = Header + Data size
- Flags control fragmentation
- Source and Destination IPs identify sender and receiver
- Data follows header as payload

Full Transcript

An IP packet begins with a header containing important fields such as Version, Header Length, Type of Service, Total Length, Identification, Flags, Fragment Offset, Time to Live, Protocol, Header Checksum, Source IP Address, Destination IP Address, and optional fields. The header length tells where the data starts. Flags control fragmentation behavior. The total length field shows the entire packet size including header and data. Source and destination IP addresses specify sender and receiver. After the header, the data payload is carried. This structure allows routers and devices to process and forward packets correctly across networks.

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