Foundations — IPv6 — address format, why needed, key differences

Before you can read a single line of the parent note IPv6 — address format, why needed, key differences, you need to earn every symbol it throws at you: bits, powers of two, hexadecimal, colons, ::, CIDR /64. This page builds each one from nothing, in the order that each depends on the last.

1. A "bit" — the atom of everything

Picture a single light switch on a wall. It has exactly two states. That "two-ness" is the seed from which every number below grows.

Why the topic needs it: the parent says "IPv4 uses a 32-bit address" and "IPv6 uses 128-bit." Those numbers are just how many switches are in the address. Everything else is counting arrangements of switches.

2. Counting combinations — where $2^n$ comes from

Follow the doubling with pictures, not formulas:

• 1 switch → 2 patterns: 0, 1.
• 2 switches → 4 patterns: 00, 01, 10, 11.
• 3 switches → 8 patterns: 000 … 111.

What the notation means: $2^n$ ("two to the power $n$") is shorthand for "multiply $2$ by itself $n$ times." The little raised number $n$ is the exponent — it counts how many switches.

See Hexadecimal and Binary Number Systems for more drills on this.

3. Grouping bits into 4 — the nibble

We could write 128 bare 0s and 1s, but no human can read 1111111010000000.... So we chop the row of switches into small chunks.

Why exactly 4? Because $16=2^4$ is a clean power of two, one nibble maps to exactly one symbol in a base-16 alphabet — no leftover states. That is the whole reason IPv6 uses hexadecimal (next section).

4. Hexadecimal — one symbol per nibble

Since a nibble has 16 possible values and hex has 16 symbols, the match is exact and one-to-one:

Why the topic needs it: this is exactly the parent's "WHY hexadecimal?" answer — hex is bit-aligned (no wasted symbols) and compact (32 characters instead of 128). Decimal would not work, because $10$ is not a power of $2$, so digits wouldn't line up on bit boundaries.

5. The colon : — the hextet separator

$\underbrace{8\text{ hextets}}\times \underset{4\text{ hex digits}}{\underbrace{16\text{ bits each}}}=128\text{ bits.}$

The colon carries no numeric meaning — it is punctuation, exactly like the dots in IPv4's 192.168.0.1. Its only job is to tell your eye where one 4-hex-digit block ends and the next begins.

6. Two zeros symbols: leading-zero drop and ::

Now that "hextet" and "colon" are earned, the parent's two compression rules are readable.

Why only once (the parent's big warning): the reader counts the visible hextets, subtracts from 8, and puts all the missing zeros where the single :: sits. With two ::, there are two gaps and no way to decide how many zeros go in each — the number becomes ambiguous, so it is illegal.

7. The slash /64 — CIDR prefix notation

$\underset{64\text{ bits}}{\underbrace{\text{network prefix}}}|\underset{64\text{ bits}}{\underbrace{\text{interface ID}}}=128\text{ bits.}$

This is the same idea as IPv4's CIDR — see IPv4 Addressing & CIDR and Subnetting. The parent's "why 64/64" reasons (MAC-derived interface IDs, SLAAC autoconfiguration) only make sense once you read /64 as "64 bits of network, 64 bits of host."

Prerequisite map

Related model context: OSI & TCP-IP Model — Network Layer, NAT (Network Address Translation), Multicast vs Broadcast vs Anycast, IP Header Structure.

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