Logical Addressing: IPv6

1. Motivation: Why IPv6?

IPv6 (Internet Protocol Version 6) was designed by the IETF (RFC 8200) to replace IPv4.

Driving Factors:

1. IPv4 Address Exhaustion: The $4.3\text{ billion}$ 32-bit IPv4 addresses were completely depleted by the exponential explosion of smartphones, IoT devices, and cloud computing.
2. Simplified Header Processing: Fixed 40-byte base header accelerates router lookup speed and hardware forwarding.
3. No Need for NAT: Provides true end-to-end direct reachability for all internet nodes.
4. Built-in Security: IPsec (Authentication and Encryption) is natively mandated in IPv6 architecture.
5. Autoconfiguration: Stateless Address Autoconfiguration (SLAAC) allows plug-and-play network connectivity without requiring a DHCP server.

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2. IPv6 Address Structure and Representation

An IPv6 address is 128 bits long (yielding $2^{128} \approx 3.4 \times 10^{38}$ unique addresses, enough to assign billions of IPs to every atom on Earth!).

• Written as 8 groups of 4 hexadecimal digits (each hex digit = 4 bits, each group = 16 bits / 2 bytes), separated by colons (:):

Address Compression & Abbreviation Rules:

1. Omission of Leading Zeros:
   • Within any 16-bit group, leading zeros can be dropped:
   • 0DB8 $\rightarrow$ DB8, 0000 $\rightarrow$ 0, 0370 $\rightarrow$ 370.

2. Double Colon (::) Zero Compression:
   • A contiguous sequence of one or more groups of all zeros can be replaced with a single double colon (::).
   • Strict Rule: The double colon can be used at most ONCE per address to avoid ambiguity:

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3. IPv6 Address Classifications

IPv6 categorizes addresses into three primary scopes:

IMPORTANT: There is NO Broadcast Address in IPv6! Broadcast functionality is entirely replaced by targeted, efficient Multicast Groups.

A. Unicast Addresses (One-to-One):

1. Global Unicast Address (GUA - 2000::/3):
   • Publicly routable on the global Internet (equivalent to public IPv4).
   • Structure: 48-bit Global Routing Prefix + 16-bit Subnet ID + 64-bit Interface ID.
2. Link-Local Address (fe80::/10):
   • Mandatory on every IPv6-enabled interface; automatically generated using the interface MAC address (EUI-64) or random generation.
   • Routable only within the local physical subnet (routers never forward link-local packets across subnets).
3. Unique Local Address (ULA - fc00::/7 or fd00::/8):
   • Routable within private enterprise intranets; unroutable on the public Internet (equivalent to RFC 1918 private IPv4).
4. Loopback Address: ::1 or ::1/128 (equivalent to 127.0.0.1 in IPv4).
5. Unspecified Address: :: or ::/128 (used when a host does not yet know its IP during bootup).

B. Multicast Addresses (One-to-Many - ff00::/8):

• Delivers a packet to all interfaces that belong to the multicast group.
• Examples:
  • ff02::1: All-Nodes multicast (replaces local subnet broadcast).
  • ff02::2: All-Routers multicast on the local link.

C. Anycast Addresses (One-to-Nearest):

• An Anycast address is assigned to multiple interfaces across different geographical locations.
• Routers deliver the packet to the physically closest / lowest-metric interface belonging to the anycast address (used for global DNS and CDN root servers).

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4. IPv6 Header Format

The base IPv6 header is streamlined to a fixed 40-byte size to eliminate variable options processing at intermediate routers:

Key Field Functions:

1. Flow Label (20 bits): Enables real-time Quality of Service (QoS) by tagging packets belonging to the same packet flow so routers handle them without deep inspection.
2. Payload Length (16 bits): Length of data payload and extension headers following the 40-byte base header.
3. Next Header (8 bits): Specifies the protocol following the IPv6 header (replaces IPv4 Protocol field; links to Extension Headers like Hop-by-Hop, Routing, Fragment, ESP/AH).
4. Hop Limit (8 bits): Counter decremented by 1 at each router hop (replaces IPv4 Time-to-Live - TTL). When it hits 0, the packet is discarded and an ICMPv6 Time Exceeded message is returned.
5. No Header Checksum!: Removed entirely to eliminate per-hop checksum recalculation latency at high router line speeds (relies on L2 CRC and L4 TCP/UDP checksums).

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5. IPv4 to IPv6 Transition Strategies

Because the entire global Internet cannot switch overnight, three primary migration mechanisms are used:

1. Dual Stack: Routers and hosts run both IPv4 and IPv6 protocol stacks concurrently.
2. Tunneling (6to4, Teredo, GRE): IPv6 packets are encapsulated inside IPv4 packet payloads to traverse legacy IPv4-only Internet backbones.
3. Translation (NAT64 / DNS64): Protocol converters translate IPv6 headers directly into IPv4 headers, allowing IPv6-only clients to communicate with IPv4-only web servers.

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