What is IPv6?

IPv6 (Internet Protocol version 6) is a new version of the internet protocol, designed as a replacement for IPv4. Its main goal is to provide a huge number of IP addresses and remove the limitations of the old version.

Short wording for an interview:

IPv6 is a 128-bit network protocol that provides addressing and routing for devices on the internet. It replaced IPv4 due to a shortage of addresses and added a simpler header, built-in security, and automatic configuration.

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#The main difference: address space size

IPv4 → 32 bits (≈ 4.3 billion addresses) IPv6 → 128 bits (≈ 3.4 × 10³⁸ addresses)

This means:

• enough addresses for every device on the planet
• NAT is no longer required
• every device can be given a globally unique address

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#The structure of an IPv6 address

An IPv6 address is 128 bits, written as 8 groups of 4 hexadecimal digits:

Example:

2001:0db8:85a3:0000:0000:8a2e:0370:7334

It can be shortened:

1. Remove leading zeros:

2001:db8:85a3:0:0:8a2e:370:7334

2. Replace consecutive zero groups with ::

2001:db8:85a3::8a2e:370:7334

The :: shorthand can be used only once.

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#Types of IPv6 addresses

#1. Global Unicast (global addresses)

The equivalent of public IPv4 addresses. Routed on the internet. Usually start with 2000::/3.

#2. Link-local

For communication within a single link (one L2 network). Automatically assigned to all interfaces:

fe80::/10

The equivalent of IPv4 APIPA (169.254.x.x), but mandatory and always in use.

#3. Unique Local Address (ULA)

Private, local addresses in IPv6 (the equivalent of RFC1918 in IPv4):

fc00::/7

#4. Multicast

A replacement for broadcast (which does not exist in IPv6):

ff00::/8

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#IPv6 does not use broadcast

Multicast is used instead, which:

• reduces load,
• makes the network more efficient.

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#Main advantages of IPv6

#1. A huge number of addresses

Every device: a unique global IP.

#2. A simplified header

The IPv6 header is simpler and processed faster by routers.

#3. SLAAC: automatic configuration

A device can obtain a global IPv6 address on its own, without DHCP.

#4. No NAT

Every host can be directly reachable (unless blocked by a firewall).

#5. Built-in IPSec support

Security at the network layer.

#6. Improved routing and multicast

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#The structure of an IPv6 packet

The IPv6 header is simpler than in IPv4:

Main fields:

• Version
• Traffic Class
• Flow Label
• Payload Length
• Next Header
• Hop Limit
• Source Address
• Destination Address

There is no:

• checksum
• fragmentation by routers (only by the host)

This speeds up processing.

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#Comparison of IPv4 and IPv6

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#Example of IPv6 addresses in a real network

Link-local address (present on all interfaces):

fe80::1c2e:4ff:feab:1234

Global address:

2001:67c:240:abcd::15

ULA (local address):

fd00:1234:abcd::1

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#Ready answer for an interview

IPv6 is a 128-bit network-layer protocol created to replace IPv4 due to a shortage of addresses. It uses hexadecimal notation, supports SLAAC, does not use broadcast, has built-in IPSec, and provides a huge address space. IPv6 addresses are divided into global unicast, link-local, ULA, and multicast.

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#Cheat sheet (5 seconds)

• IPv6 = 128 bits, hexadecimal addresses.
• Does not use broadcast → multicast.
• Addresses: global, link-local (fe80::), ULA (fc00::), multicast (ff00::).
• Supports SLAAC and built-in IPSec.
• Solves the problem of IPv4 shortage.