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IPv6 Primer

A practical introduction to IPv6 for developers and network administrators.

What is IPv6?

IPv6 (Internet Protocol version 6) is the latest version of the Internet Protocol, designed to replace IPv4. It provides:

  • Vastly larger address space: 128-bit addresses vs 32-bit in IPv4
  • No NAT required: Every device can have a globally routable address
  • Built-in security: IPSec is mandatory (optional in IPv4)
  • Simpler routing: More hierarchical addressing
  • Auto-configuration: SLAAC (Stateless Address Autoconfiguration)

IPv6 Address Format

Structure

IPv6 addresses are 128 bits written as 8 groups of 4 hexadecimal digits:

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

Compression Rules

  1. Leading zeros can be omitted:

    2001:0db8:85a3:0000 → 2001:db8:85a3:0

  2. Consecutive zeros can be replaced with :: (only once):

    2001:0db8:0000:0000:0000:0000:0000:0001 → 2001:db8::1

Examples

Uncompressed Compressed
0000:0000:0000:0000:0000:0000:0000:0001 ::1
fe80:0000:0000:0000:0000:0000:0000:0001 fe80::1
2001:0db8:0000:0042:0000:8a2e:0370:7334 2001:db8:0:42:0:8a2e:370:7334

Address Types

Unicast Addresses

Global Unicast (2000::/3)

  • Globally routable addresses
  • Similar to public IPv4 addresses
  • Example: 2001:db8:1234::1

Link-Local (fe80::/10)

  • Only valid on local network segment
  • Auto-configured on every IPv6 interface
  • Not routable beyond local link
  • Example: fe80::1 (often with zone: fe80::1%eth0)

Unique Local (fc00::/7)

  • Private addresses (like IPv4 RFC1918)
  • Prefix: fd00::/8 for locally assigned
  • Not globally routable
  • Example: fd12:3456:789a::1

Loopback (::1/128)

  • Localhost address
  • Equivalent to 127.0.0.1 in IPv4

Unspecified (::/128)

  • All zeros
  • Used to indicate absence of address
  • Never assigned to interface

Multicast (ff00::/8)

  • One-to-many communication
  • No broadcast in IPv6 (multicast instead)
  • Examples:
    • ff02::1 - All nodes on local link
    • ff02::2 - All routers on local link
    • ff02::1:ff00:0/104 - Solicited-node multicast

Prefix Notation (CIDR)

IPv6 uses CIDR notation like IPv4:

2001:db8::/32
  ↑        ↑
  prefix   length

Common Prefix Lengths

  • /128 - Single host
  • /64 - Single subnet (standard)
  • /56 - Typical home user allocation
  • /48 - Typical site/organization
  • /32 - ISP allocation
  • /8 - Large regional allocation

Why /64 is Standard

  • Required for SLAAC
  • Allows 64-bit interface ID (EUI-64)
  • 2^64 addresses per subnet (18 quintillion!)

Address Assignment Methods

1. Static Configuration

Manually configure address and prefix:

ip -6 addr add 2001:db8::1/64 dev eth0

2. SLAAC (Stateless Address Autoconfiguration)

  • Automatic configuration using Router Advertisements
  • Interface creates address from prefix + interface ID
  • Most common for hosts

3. DHCPv6 (Stateful)

  • Similar to DHCP in IPv4
  • Provides more control (DNS, etc.)
  • Can work alongside SLAAC

4. Privacy Extensions (RFC 4941)

  • Generates temporary random addresses
  • Improves privacy (harder to track devices)
  • Changes addresses periodically

Special Addresses

Address Purpose
::1 Loopback
:: Unspecified
fe80::/10 Link-local
ff00::/8 Multicast
2001:db8::/32 Documentation (examples)
2002::/16 6to4 transition
fd00::/8 Unique local

IPv6 in URLs

IPv6 addresses in URLs must be enclosed in brackets:

http://[2001:db8::1]/
http://[2001:db8::1]:8080/path
https://[fe80::1%eth0]/

Neighbor Discovery Protocol (NDP)

Replaces ARP from IPv4:

  • Router Solicitation/Advertisement: Find routers
  • Neighbor Solicitation/Advertisement: Find neighbors (like ARP)
  • Redirect: Inform of better routes

Uses ICMPv6 messages.

Transition Mechanisms

Dual Stack

  • Run IPv4 and IPv6 simultaneously
  • Most common approach
  • Applications choose which to use

Tunneling

  • 6in4: IPv6 over IPv4 tunnel
  • 6to4: Automatic tunneling
  • Teredo: For hosts behind NAT

Translation

  • NAT64: IPv6-only to IPv4 communication
  • DNS64: Synthesize AAAA records

IPv6 Headers

Simplified Header

IPv6 header is simpler than IPv4:

  • Fixed 40-byte header
  • No header checksum
  • No fragmentation by routers
  • Extension headers for optional features

Extension Headers

  • Routing
  • Fragment
  • Authentication (AH)
  • Encapsulation (ESP)
  • Hop-by-hop options
  • Destination options

Subnetting Examples

Example 1: Enterprise Network

You receive: 2001:db8::/32

Allocation:

HQ:          2001:db8:0::/48
Branch 1:    2001:db8:1::/48
Branch 2:    2001:db8:2::/48
...
Branch 65535: 2001:db8:ffff::/48

Within HQ:

Engineering: 2001:db8:0:1::/64
Sales:       2001:db8:0:2::/64
IT:          2001:db8:0:3::/64
...

Example 2: Data Center

You have: 2001:db8::/32

/32         Provider allocation
/40         Region (256 regions)
/48         Data center (256 DCs per region)
/56         Customer (256 customers per DC)
/64         Subnet (256 subnets per customer)

Best Practices

1. Addressing Plan

  • Plan hierarchy before deployment
  • Use consistent patterns
  • Document allocations
  • Leave room for growth

2. Use Standard Prefixes

  • /64 for end-user subnets
  • /48 for sites
  • Don't use prefixes longer than /64 for SLAAC

3. Security

  • Still need firewalls (IPv6 doesn't mean "no firewall")
  • Be aware of extension headers
  • Monitor ICMPv6 (needed for functionality)
  • Watch for ND attacks

4. Monitoring

  • Track address usage
  • Monitor ND cache
  • Watch for rogue RAs
  • Log unusual traffic

5. Documentation

  • Document address plan
  • Note special allocations
  • Keep DNS updated
  • Maintain IPAM database

Common Misconceptions

❌ "IPv6 is secure so I don't need a firewall"

FALSE: IPv6 includes IPSec, but firewalls are still necessary.

❌ "I can use any address in fc00::/7"

PARTIAL: Use fd00::/8. Generate random global ID.

❌ "I should conserve IPv6 addresses"

FALSE: Address space is huge. Use /64 liberally.

❌ "NAT provides security"

FALSE: NAT provides obscurity, not security. Use firewalls.

❌ "I can disable ICMPv6"

FALSE: ICMPv6 is essential for IPv6 operation (NDP, PMTUD).

Quick Reference

Address Classes

::1/128                 Loopback
::/128                  Unspecified
fe80::/10               Link-local
fc00::/7                Unique local (ULA)
2000::/3                Global unicast
ff00::/8                Multicast
2001:db8::/32           Documentation

Tools

# Show IPv6 addresses
ip -6 addr show

# Show IPv6 routes
ip -6 route show

# Show neighbors
ip -6 neigh show

# Ping IPv6
ping6 2001:db8::1

# Trace route
traceroute6 2001:db8::1

# DNS lookup
dig AAAA example.com

Python Quick Start

from ipv6tools import IPv6Address, IPv6Network

# Create address
addr = IPv6Address("2001:db8::1")
print(addr.is_global)  # True

# Create network
net = IPv6Network("2001:db8::/32")
print(net.contains("2001:db8::1"))  # True

Learning Resources

RFCs

Websites

Books

  • "IPv6 Fundamentals" by Rick Graziani
  • "IPv6 Address Planning" by Tom Coffeen

Next Steps

  1. Read the Tutorial
  2. Try the Examples
  3. Use the Web Tools
  4. Practice with the CLI Tools