--- title: "IPv4/IPv6 coexistence and dual stack" subject: "Computer Networks and Security" chapter: 7 key: "semester_7/computer_networks_and_security:dual-stack-coexistence" source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/dual-stack-coexistence site: BCT Engineering --- # IPv4/IPv6 coexistence and dual stack Computer Networks and Security · Chapter 7: Introduction to IPV6 - Asked in: 2073 Chaitra, 3+5 marks - Video: [Coexistence with IPv4](https://www.youtube.com/watch?v=xaj_olnAAj0) — ITFreeTraining - 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/dual-stack-coexistence - Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-7 ## Question 'IPv4 and IPv6 coexist': what does this mean? Explain the dual stack approach with an appropriate figure. ## Short answer IPv4 and IPv6 coexist by operating simultaneously during transition. Dual stack means a host runs both protocol stacks, obtains an IPv4 and IPv6 address, and uses DNS (A or AAAA record) to select the version for communication. ## Answer #### Coexistence of IPv4 and IPv6 "IPv4 and IPv6 coexist" means that for many years both protocols operate side by side on the Internet, because IPv6 is not backward compatible and billions of IPv4 devices cannot be replaced at once. Networks, hosts and applications using either version must continue to communicate during the transition period. #### Dual stack approach In the dual stack method, a host or router has **both an IPv4 stack and an IPv6 stack** implemented on the same interface, and therefore has both an IPv4 address and an IPv6 address. ```text Application (browser) | +-----+------+ | TCP / UDP | +--+------+--+ | | IPv4 IPv6 +--+---+ Ethernet / Wi-Fi ``` #### Working 1. The application asks DNS for the destination. If DNS returns an **AAAA** record, the host uses IPv6; if only an **A** record is returned, it uses IPv4. 2. The IPv4 packet or IPv6 packet is then built by the corresponding stack. 3. Routers run both protocols and both routing protocols (e.g. OSPFv2 and OSPFv3), and forward each packet according to its version. ```text IPv4 host --- [dual stack router] --- IPv6 host dual-stack host talks to both ``` #### Advantages and limitations - Simple, no translation, full features of both protocols, gradual deployment. - Each device needs two addresses and two sets of configuration and security rules; memory and processing are higher; still needs IPv4 addresses, so the shortage remains. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why must IPv4 and IPv6 coexist for years? A. IPv6 is backward compatible so no change is needed B. IPv4 is more secure C. IPv6 is not backward compatible and billions of IPv4 devices cannot be replaced at once D. DNS cannot support IPv6 Answer: C. Both run side by side during the transition because IPv6 is not backward compatible with IPv4. (Key point: Coexistence meaning) 2. How does a dual-stack host choose which protocol to use? A. It alternates packets B. DNS returns AAAA, use IPv6; only A, use IPv4 C. The router decides D. It always uses IPv6 Answer: B. If DNS returns an AAAA record the host uses IPv6; if only an A record, IPv4. (Key point: Choosing the stack) 3. How do dual-stack routers forward packets? A. Run both protocols and routing protocols (e.g. OSPFv2 and OSPFv3), forwarding by packet version B. Convert all to IPv6 C. Only forward IPv6 D. Drop IPv4 packets Answer: A. Routers run both stacks and both routing protocols and forward each packet according to its version. (Key point: Dual-stack routers) 4. Which is a limitation of dual stack? A. It needs translation gateways B. It cannot work on Ethernet C. The IPv4 shortage remains because IPv4 addresses are still needed D. It loses IPv6 features Answer: C. Each device needs both addresses, more configuration and resources, and IPv4 addresses are still required. (Key point: Limitation) ## Related in Computer Networks and Security - [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× - [Compare IPv6/IPv4 headers; migration](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aipv6-ipv4-header-migration) — 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 - [What is IPv6; interoperability methods](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aipv6-interoperability-methods) — Computer Networks and Security, chapter 7 - [IPv6 datagram format](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aipv6-datagram-format) — Computer Networks and Security, chapter 7, asked 2× ## Related in other subjects - [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 - [RFC and IPv4/IPv6 fragmentation](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Arfc-fragmentation-ipv4-ipv6) — Internet and Intranet, chapter 2 - [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