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

Concept Flow - IP packet structure

Start: IP Packet

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Header Fields

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Version

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Header Length

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Type of Service

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Total Length

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Identification

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Flags

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Fragment Offset

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Time to Live

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Protocol

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Header Checksum

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Source IP Address

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Destination IP Address

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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.