---
title: "Compare IPv6/IPv4 headers; migration"
subject: "Computer Networks and Security"
chapter: 7
key: "semester_7/computer_networks_and_security:ipv6-ipv4-header-migration"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/ipv6-ipv4-header-migration
site: BCT Engineering
---

# Compare IPv6/IPv4 headers; migration

Computer Networks and Security · Chapter 7: Introduction to IPV6

- Asked in: 2072 Chaitra, 4+4 marks; 2070 Asar, 4+4 marks (asked 2 times)
- Video: [L58: IPv4 to IPv6 Transition and Difference b/w IPv4 and IPv6 | Data Communication Network Lectures](https://www.youtube.com/watch?v=DMnCkbcJ-qw) — Easy Engineering Classes
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/ipv6-ipv4-header-migration
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-7

## Question

Compare the header fields of IPv6 and IPv4. Which method do you suggest for the migration from IPv4 to IPv6 and why?

## Short answer

IPv6 header is fixed 40 bytes with 8 fields vs IPv4's 20-60 bytes with 12 fields. Key changes: removed IHL, checksum, fragmentation fields; renamed TOS to Traffic Class, TTL to Hop Limit, Protocol to Next Header; added Flow Label. Dual stack with tunneling recommended for migration.

## Answer

#### Comparison of IPv4 and IPv6 headers
IPv4 header is 20 to 60 bytes with 12 fields plus options. IPv6 header is a fixed 40 bytes with 8 fields.

| IPv4 field | IPv6 equivalent |
|---|---|
| Version (4) | Version (6) |
| IHL (header length) | Removed (header is fixed 40 B) |
| Type of service | Renamed Traffic class (8 bits) |
| Total length | Renamed Payload length (16 bits, excludes the header) |
| Identification, Flags, Fragment offset | Removed; moved to the Fragment extension header |
| TTL | Renamed Hop limit |
| Protocol | Renamed Next header (also points to extension headers) |
| Header checksum | Removed |
| Source address 32 bits | Source address 128 bits |
| Destination address 32 bits | Destination address 128 bits |
| Options + padding | Removed; extension headers |
| (none) | **Flow label** (20 bits), new |

```text
IPv6: Ver | TrafficClass | FlowLabel
      PayloadLen | NextHdr | HopLimit
      Source address (128)
      Destination address (128)
```

#### Suggested migration method
**Dual stack, with tunneling where an IPv4-only network separates IPv6 sites.**

Reasons:
1. Both protocols work at the same time, so no existing service is broken; hosts and routers are upgraded gradually.
2. No packet conversion is needed, so there is no loss of header information such as flow label and no performance cost.
3. Applications choose IPv6 or IPv4 automatically using DNS.
4. Tunneling is a quick, low-cost way to link IPv6 islands over the existing IPv4 Internet.
5. Header translation is kept only for IPv6-only to IPv4-only communication because it loses features and breaks end-to-end behaviour.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. TTL and Protocol in IPv6 are renamed:
   A. Hop limit and Next header
   B. Flow label and Traffic class
   C. Payload and Checksum
   D. Hop and Class
   Answer: A. TTL -> Hop limit; Protocol -> Next header. (Key point: Renamed fields)

2. Which is removed in IPv6?
   A. Header checksum
   B. Source address
   C. Version
   D. Hop limit
   Answer: A. Header checksum, IHL and fragmentation fields are removed. (Key point: Removed)

3. Suggested migration method and reason:
   A. Dual stack with tunneling; both work together without breaking services
   B. Header translation only
   C. Flag day switch
   D. Drop IPv4 immediately
   Answer: A. Gradual, no conversion loss. (Key point: Migration)

## Related in Computer Networks and Security

- [TCP header and its fields](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Atcp-header-structure) — Computer Networks and Security, chapter 5, asked 2×
- [IPv4 drawbacks solved by IPv6](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aipv4-drawbacks-ipv6-advantages) — Computer Networks and Security, chapter 7, asked 2×
- [Factors for IPv6; IPv4 to IPv6 transition](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aipv6-factors-transition) — Computer Networks and Security, chapter 7, asked 2×
- [IPv4 problems, IPv6 fixes, transition](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aipv6-problems-solutions-transition) — Computer Networks and Security, chapter 7
- [IPv4/IPv6 coexistence and dual stack](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Adual-stack-coexistence) — Computer Networks and Security, chapter 7
- [Header translation and tunneling](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aheader-translation-tunneling) — Computer Networks and Security, chapter 7

## Related in other subjects

- [IPv6 migration; IPv4 vs IPv6 packet structure](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aipv6-migration-router-packet) — Internet and Intranet, chapter 2
- [IPv4 vs IPv6 datagram fragmentation](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aipv4-ipv6-fragmentation) — Internet and Intranet, chapter 2, asked 2×
- [TCP/IP applications; IPv4 vs IPv6 routing](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Atcpip-application-ipv4-ipv6-routing) — Internet and Intranet, chapter 2
- [IPv6 features, header format, RFC streams](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aipv6-features-header-rfc) — Internet and Intranet, chapter 2