IPv6

IPv6 moves packets across interconnected networks. It gives each packet a source address, a destination address, and enough control information for routers to forward it. It does not guarantee delivery, restore lost packets, or preserve order.

IPv6 succeeds IPv4 at the network layer. The two protocols serve the same basic role, but IPv6 is not IPv4 with longer notation. It uses 128-bit addresses, a different base header, ICMPv6-based Neighbor Discovery, and source-only fragmentation.

Start with one packet

An IPv6 packet contains a fixed-format base header followed by a payload. The payload can begin with an extension header or an upper-layer protocol such as TCP, UDP, or ICMPv6.

The base header contains:

• Version, traffic class, and flow label
• Payload length
• Next header
• Hop limit
• A 128-bit source address
• A 128-bit destination address

The base header is 40 octets. It has no header checksum. The next-header value identifies either another IPv6 header or the upper-layer protocol that follows.

Extension headers carry optional internet-layer information. A node follows the next-header chain to interpret them. Not every device processes every extension header. The destination node normally handles them, with defined exceptions such as Hop-by-Hop Options.

Read addresses as hexadecimal groups

An IPv6 address has eight 16-bit fields in its full text form. Each field uses hexadecimal.