IPv6 Fundamentals

IPv4 uses 32-bit addresses — about 4.3 billion. IPv6 uses 128-bit addresses — 340 undecillion ($3.4 \times 10^\{38\}$). That’s enough for every grain of sand on Earth to have its own IP.

Address Format

IPv6 is written as 8 groups of 4 hex digits, separated by colons:

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

Shortening rules:

1. Leading zeros in a group can be omitted → 0370 → 370
2. One contiguous group of all-zero groups can be replaced with ::
   • 2001:0db8:85a3:0000:0000:8a2e:0370:7334 → 2001:db8:85a3::8a2e:370:7334

::1 is the IPv6 loopback (equivalent to 127.0.0.1).

Key Differences from IPv4

Feature	IPv4	IPv6
Address size	32-bit	128-bit
Notation	Dotted decimal (192.168.1.1)	Colon hex (2001:db8::1)
NAT	Required (address shortage)	Not needed
Security (IPsec)	Optional	Mandatory
Broadcast	Yes	No (multicast + anycast)
Header processing	Complex (checksum, options)	Simple (fixed header, no checksum)
Fragmentation	Routers can fragment	Only the sender fragments
Address config	DHCP (manual or dynamic)	SLAAC (Stateless Address Autoconfiguration)

IPv6 Header

Fixed 40-byte header (simpler than IPv4’s variable-length header):

Version (4) | Traffic Class (8) | Flow Label (20)
Payload Length (16) | Next Header (8) | Hop Limit (8)
Source Address (128)
Destination Address (128)

No checksum (higher layers handle it), no fragmentation fields (handled via Path MTU Discovery).

Transition Mechanisms

Method	How it works
Dual Stack	Devices run both IPv4 and IPv6 stacks simultaneously
Tunneling	IPv6 packets encapsulated inside IPv4 packets (6to4, Teredo)
Translation	NAT64/PAT — translates between IPv6 and IPv4

Dual stack is the most common: if IPv6 works, use it; otherwise fall back to IPv4.