IPv6

Computer Network & Network Security System — Network Layer, NEC licence examination syllabus (Nepal Engineering Council).

IPv6: the internet's replacement address book

4.3 billion IPv4 addresses ran out. IPv6 offers roughly 340 undecillion — deliberately, absurdly overkill.

📦 IPv6 Header

Address size128 bits (vs IPv4's 32 bits), written in hexadecimal groups (e.g. 2001:0db8::1).

Simplified headerFewer fields than IPv4's header, designed for faster router processing.

Extension headersOptional headers (for security, routing options, fragmentation, etc.) chained after the main header only when actually needed — instead of IPv4's approach of always reserving space for options that are usually unused.

Transitioning from IPv4 to IPv6

🔀 Three Transition Strategies

Dual stackA device runs both IPv4 and IPv6 simultaneously, using whichever the destination supports.

TunnelingIPv6 packets get wrapped inside IPv4 packets to cross IPv4-only network segments.

Header translationActively rewrites headers between the two formats at the boundary between an IPv4 and IPv6 network.

IPv6 multicasting is built into the protocol design from the ground up (IPv4 mostly bolted multicast on later) — a single IPv6 address prefix is dedicated entirely to multicast use.

💡 One-liner: "Why does IPv6 use extension headers instead of a fixed options field like IPv4?" → Keeps the main header small and fast to process for the common case, only adding complexity when a specific feature is actually needed.

How much bigger 128 bits actually is

"340 undecillion" conveys nothing useful. Two comparisons make it concrete.

IPv4: 2³² ≈ 4.29 × 10⁹ addresses IPv6: 2¹²⁸ ≈ 3.40 × 10³⁸ addresses The ratio is 2⁹⁶ ≈ 7.9 × 10²⁸ — IPv6 does not have four times as many addresses, it has that many TIMES as many. Spread over the Earth's surface, that is roughly 6.7 × 10²³ addresses per square metre.

The size is deliberate excess, and the excess is the point. Address space is spent freely to make everything else simpler — every subnet is a /64, wasting 18 quintillion addresses per network, because a fixed boundary means no subnet arithmetic and no agonising over allocation sizes. IPv4's problem was never really scarcity, it was the administrative cost of allocating a scarce resource carefully.

What the simplified header buys

📦 Three deliberate removals

No header checksumRemoved entirely. The link layer already checks frames and the transport layer checks end to end, so recomputing it at every router was pure cost — a router must recompute it because it decrements the hop limit.

No router fragmentationOnly the sender may fragment. A router that meets a too-large packet discards it and reports back, so routers do no per-packet reassembly bookkeeping.

Fixed 40-byte headerOptions moved into optional extension headers, so the common case is a fixed size a router can parse without examining lengths.

💡 Every one of these makes the router's job cheaper, which is the design goal: a router touches every packet, so work removed from the header is multiplied by the entire internet's traffic. Work moved to the endpoints is done once per packet by machines that are not the bottleneck.

Why adoption took decades

IPv6 is not backward compatible — an IPv6-only host cannot talk to an IPv4-only host, because the address does not fit in the field. So the two run side by side, with hosts and routers supporting both, and neither side can switch off until essentially everyone has moved.

The other reason is that NAT worked too well. Private addressing let one public address front thousands of machines, which relieved the pressure that would otherwise have forced migration. It is a workaround that removed the urgency of the fix — at the cost of end-to-end addressability, which is why protocols like AH break through NAT and why peer-to-peer connections need workarounds.

Syllabus points

IPv6 packet/header format
Extension headers
Transition from IPv4: dual stack, tunneling, header translation
IPv6 multicasting

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