Learning Objectives

• Understand the IPv6 header structure and differences from IPv4
• Visualize Neighbor Discovery Protocol operation
• Compare IPv6 routing protocols with their IPv4 counterparts
• Match IPv6 concepts to their IPv4 equivalents

The IPv6 Header

IPv6 simplifies the packet header to speed up forwarding. Fixed at 40 bytes (compared to IPv4's variable 20-60 bytes), the IPv6 header removes checksums, fragmentation fields, and options — pushing these to extension headers.

Key differences from IPv4:

• No header checksum — Upper layers handle integrity; improves forwarding speed
• No fragmentation — Hosts use Path MTU Discovery; routers no longer fragment
• No options — Extension headers replace the variable-length options field
• Hop Limit replaces TTL — Same function, more accurate name

Neighbor Discovery Protocol (NDP)

IPv6 replaces ARP with Neighbor Discovery Protocol (NDP) , using ICMPv6 messages. NDP handles address resolution, router discovery, and stateless address autoconfiguration (SLAAC).

NDP messages:

• Router Solicitation (RS) — Hosts request router information
• Router Advertisement (RA) — Routers advertise prefix, MTU, and default gateway
• Neighbor Solicitation (NS) — Resolve a neighbor's MAC address (like ARP)
• Neighbor Advertisement (NA) — Response to NS with MAC address
• Redirect — Router tells host about a better next-hop

IPv6 Routing Protocols

IPv6 uses the same routing protocols as IPv4, adapted with minor modifications:

| Protocol | IPv4 Version | IPv6 Version | Key Difference | |----------|-------------|-------------|----------------| | OSPFv2 | OSPF for IPv4 | OSPFv3 | Runs on link, not subnet; uses link-local addresses | | BGP-4 | BGP for IPv4 | MP-BGP | New address family: IPv6 unicast | | EIGRP | EIGRP for IPv4 | EIGRP for IPv6 | Similar operation; uses IPv6 transport | | RIP | RIPng | RIP next generation | Same distance-vector approach |

OSPFv3, for example, is nearly identical to OSPFv2 but:

• Uses link-local addresses for neighbor discovery
• Removes per-subnet semantics — adjacencies form per-link
• Carries multiple address families (IPv6 and optionally IPv4)

Key Takeaways

• IPv6 header is fixed at 40 bytes with no checksum, no fragmentation, and no options
• NDP replaces ARP using ICMPv6 messages (RS, RA, NS, NA)
• All major routing protocols have IPv6 equivalents (OSPFv3, MP-BGP, RIPng)
• IPv6 eliminates broadcast in favor of multicast and anycast
• Understanding the IPv4-to-IPv6 mapping is essential for mixed-protocol networks