--- title: "IPv6 migration; IPv4 vs IPv6 packet structure" subject: "Internet and Intranet" chapter: 2 key: "semester_8/internet_and_intranet:ipv6-migration-router-packet" source: https://www.bctengineer.com/q/semester_8/internet_and_intranet/ipv6-migration-router-packet site: BCT Engineering --- # IPv6 migration; IPv4 vs IPv6 packet structure Internet and Intranet · Chapter 2: Internet Protocol Overview - Asked in: 2070 Bhadra, 4+6 marks - Video: [Internet Protocol - IPv4 vs IPv6 as Fast As Possible](https://www.youtube.com/watch?v=aor29pGhlFE) — Techquickie - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/internet_and_intranet/ipv6-migration-router-packet - Whole chapter: https://www.bctengineer.com/md/semester_8/internet_and_intranet/chapter-2 ## Question Why the world has decided to migrate to new internet addressing scheme IPv6? Compare IPv4 and IPv6 router with their packet structure. ## Short answer Migration is needed because IPv4 addresses are exhausted, NAT breaks end-to-end, routing tables are large, and IPv6 adds autoconfiguration, security, and efficiency. IPv4 routers fragment, use ARP and DHCP, and have variable 20-60 byte headers with checksum and TTL. IPv6 routers never fragment, use NDP and SLAAC, and have fixed 40-byte headers with flow label and 128-bit addresses. ## Answer #### Why migrate to IPv6 1. **IPv4 address exhaustion:** IPv4 has 2^32 (about 4.3 billion) addresses. IANA gave out its last blocks in 2011 and the RIRs followed, while billions of phones, laptops and IoT devices need addresses. 2. **NAT problems:** NAT, the workaround, breaks end-to-end connectivity and complicates peer-to-peer apps, VoIP and security (IPsec). 3. **Large routing tables:** poor aggregation of IPv4 blocks makes backbone routing tables huge; IPv6 is hierarchical. 4. **Auto-configuration:** IPv4 needs DHCP or manual set-up; IPv6 hosts configure themselves. 5. **Security and QoS:** IPsec support is built in, and the flow label supports QoS. 6. **Efficiency:** a simpler header, no router fragmentation and no checksum speed up forwarding. 7. **Mobility and growth:** better support for mobile devices and future services. #### IPv4 router vs IPv6 router | Basis | IPv4 router | IPv6 router | |---|---|---| | Header processing | Variable header, recomputes checksum, decrements TTL | Fixed header, no checksum, decrements Hop Limit | | Fragmentation | Fragments if needed | Never fragments; sends ICMPv6 Packet Too Big | | Address resolution | ARP | Neighbour Discovery (NDP) | | Host configuration | DHCP | Router Advertisement (SLAAC), DHCPv6 | | Routing protocols | RIPv2, OSPFv2, BGP-4 | RIPng, OSPFv3, MP-BGP | | Broadcast | Forwards or blocks broadcasts | No broadcast; uses multicast | | Table size | Large | Smaller | #### Packet structure comparison | IPv4 header | IPv6 header | |---|---| | 20 to 60 bytes, variable (Options field) | Fixed 40 bytes | | Version, IHL, ToS, Total Length | Version, Traffic Class, Payload Length | | Identification, Flags, Fragment Offset | Removed; moved to Fragment extension header | | TTL | Hop Limit | | Protocol | Next Header | | Header Checksum | Removed | | Source/Destination 32 bit | Source/Destination 128 bit | | Options, Padding | Extension headers | | (none) | Flow Label (new, 20 bits) | ```text IPv4: [Ver|IHL|ToS|Total Len] [ID | Flags | Frag Off ] [TTL|Proto| Checksum ] [Source 32][Dest 32][Options] IPv6: [Ver|TClass|Flow Label ] [Payload Len|NextHdr|Hop] [Source 128][Dest 128] ``` IPv6 has 8 fields against 12 in IPv4. Both are carried inside the link-layer frame (e.g. Ethernet type 0x0800 for IPv4 and 0x86DD for IPv6). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main reason the world is migrating to IPv6? A. IPv4 is slower on all networks B. IPv4 has only about 4.3 billion (2^32) addresses and they have been exhausted C. IPv4 cannot carry TCP D. IPv4 has no header Answer: B. IANA and the RIRs ran out of IPv4 blocks while billions of devices need addresses. (Key point: IPv4 exhaustion) 2. Why is NAT, the workaround for IPv4 shortage, a problem? A. It breaks end-to-end connectivity and complicates peer-to-peer, VoIP and IPsec B. It makes addresses longer C. It removes the need for routers D. It disables DNS Answer: A. NAT hides hosts behind shared addresses, so end-to-end connectivity and applications like VoIP and IPsec suffer. (Key point: NAT problems) 3. Which IPv4 header items are removed from the IPv6 base header? A. Source and Destination Address B. Version and Payload Length C. Header Checksum and the fragmentation fields (ID, Flags, Offset) D. TTL and Protocol Answer: C. Checksum is dropped and fragmentation info moves to the Fragment extension header; TTL and Protocol become Hop Limit and Next Header. (Key point: Fields removed from IPv6 header) 4. How does an IPv6 router differ from an IPv4 router? A. It fragments packets more often B. It never fragments, uses Neighbour Discovery instead of ARP and has no broadcast C. It uses ARP and broadcasts D. It recomputes a checksum at each hop Answer: B. IPv6 routers send ICMPv6 Packet Too Big instead of fragmenting, use NDP rather than ARP and use multicast. (Key point: Router differences) ## Related in Internet and Intranet - [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 role and IPv6 addressing](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Arfc-development-ipv6-addressing) — 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 - [Content filtering vs packet filtering](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Acontent-filtering-packet-filtering) — Internet and Intranet, chapter 5 - [Internet vs Intranet and DNS need](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Ainternet-intranet-compare-dns) — Internet and Intranet, chapter 1 ## Related in other subjects - [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× - [Internet structure: repeater, bridge, router, gateway](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ainternet-structure-devices) — Telecommunications, chapter 8 - [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×