IPv6 // RFC 8200

The IPv6 header is always 40 bytes. The first 32 bits pack Version, Traffic Class and Flow Label; then come Payload Length, Next Header, Hop Limit, and the 128-bit source and destination addresses. There is no IHL, no header checksum and no base-header options field.

Version, Traffic Class and Flow Label share the first 32 bits. Traffic Class is the same DSCP/ECN octet used by IPv4 Differentiated Services; Flow Label is a separate 20-bit identifier intended for flow classification and load balancing. It is not a port and it does not identify a connection.

0 · 0x00

0x12345

0x60012345

An IPv6 address is 128 bits, written as eight 16-bit hexadecimal groups. Leading zeros in a group are removed and the longest run of zero groups is compressed with ::. Only one :: is allowed. The wire format is always the full 16 bytes; the shortened text form is only a display convention.

IPv6 moves optional processing into separate extension headers. Each one starts with a Next Header byte and a length field, then type-specific data. Only Hop-by-Hop must be first; the usual order is Hop-by-Hop, Destination Options, Routing, Fragment, AH, ESP, a final Destination Options header, then the upper-layer protocol.

Common options

Build a structurally valid IPv6 base header and append a payload. Payload Length is calculated from the bytes you supply, so it includes any extension headers you paste or generate. The optional presets create a TCP, UDP, ICMPv6, Hop-by-Hop or Fragment example; transport checksums are calculated for the current source and destination when a preset is loaded.

no bytes built yet

Paste a raw IPv6 packet or an Ethernet II frame with EtherType 0x86DD. The decoder follows the Next Header chain through Hop-by-Hop, Routing, Fragment, AH and Destination Options headers, reports each length and offset, and stops at the first upper-layer protocol. It also detects RFC 2675 Jumbo Payload options.

• Payload Length includes extension headers. It is not the length of the upper-layer payload alone. Subtract the chain lengths to find the transport payload. A zero value is meaningful: empty packet or jumbogram with a Jumbo Payload option.
• Hop Limit is not TTL. It counts hops, not seconds, and is decremented on every forwarding router. Initial values are convention, not protocol constants. Zero produces ICMPv6 Time Exceeded.
• Fragmentation belongs to the source. IPv6 routers do not fragment packets; they send ICMPv6 Packet Too Big when the packet exceeds the path MTU. The Fragment header uses a 32-bit identification and an offset in 8-byte units.
• Address text is not wire format. The wire is always 16 bytes. RFC 5952 canonical text compresses the longest zero run, lowercases hex and omits leading zeros. Zone identifiers such as %eth0 are local scope syntax.
• No broadcast. IPv6 uses multicast and anycast. All-nodes multicast is ff02::1; solicited-node multicast is ff02::1:ff00:0/104. ARP is replaced by Neighbor Discovery over ICMPv6.
• Extension chains need limits. A parser must bound the number of extension headers it follows, validate every length before slicing, and stop at ESP because the next header is encrypted.
• Filtering ICMPv6 breaks IPv6. Neighbor Discovery, Router Advertisement, Packet Too Big and Time Exceeded are not optional noise. Blanket ICMP drop policies are a common cause of IPv6 black holes.
• Extension headers are not IPv4 options. They are chained through Next Header values and counted in Payload Length; AH uses 4-octet length units, while Hop-by-Hop, Routing and Destination Options use 8-octet units.

References

• RFC 8200IPv6 specification
• RFC 4291IPv6 addressing architecture
• RFC 5952IPv6 address text representation
• RFC 4007Scoped IPv6 addresses
• RFC 8201Path MTU Discovery
• RFC 6437IPv6 flow label
• RFC 2675IPv6 jumbograms
• RFC 4443ICMPv6
• RFC 4861Neighbor Discovery
• RFC 8754Segment Routing Header