Quiz Entry - updated: 2026.07.14
Comparing the IPv4 and IPv6 headers — which IPv4 fields were removed in IPv6, and why?
IPv6 dropped the Header Checksum and the fragmentation fields (Flag and Fragment Offset) to simplify the header and speed up router processing.
Fields removed from the IPv4 header in IPv6:
| Removed field | Why it was dropped |
|---|---|
| Header Checksum | Upper-layer (TCP/UDP) and Layer 2 already provide error checking, so per-hop recomputation was wasteful |
| Flags / Fragment Offset | IPv6 routers never fragment; fragmentation is handled only by the source host (via an extension header), so these fields are not needed in the main header |
Key consequences:
- The IPv6 header is fixed at 40 bytes and simplified — but not smaller than IPv4's variable header.
- Removing per-hop checksum recomputation means routers do less work per packet, improving forwarding performance.
- The DS/QoS field survives as Traffic Class, and TTL is renamed Hop Limit; Flow Label is new in IPv6.
Context: The design theme is "push complexity to the endpoints." By moving fragmentation to the source and removing the checksum, IPv6 keeps the fast path (routers) lean.
Go deeper:
RFC 8200 §3 — IPv6 Header Format — the simplified header that dropped the checksum and fragmentation fields.- Wikipedia — IPv6 packet — side-by-side view of what IPv6 keeps, renames, and drops.