IPv6 Address

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• pool of available IPv4 public addresses is not very large, compared to the number of devices that need to connect to the Internet
• IP version 6 (IPv6) is intended to replace IPv4 completely

IPv6 address is a 128-bit number and so can express exponentially more address values than the 32-bit number used in IPv4

IPv6 Notation

• written in hexadecimal notation
• One hex digit can represent a four-bit binary value (a nibble)
• To express a 128-bit IPv6 address in hex, the binary address is divided into eight double-byte (16-bit) values delimited by colons
  • E.g., 2001:0db8:0000:0000:0abc:0000:def0:1234
• To shorten how this is written and typed:
  • where a double byte contains leading zeros, they can be ignored
  • one contiguous series of zeroes can be replaced by a double colon place marker
  • E.g., above becomes 2001:db8::abc:0:def0:1234

IPv6 Network Prefixes

• An IPv6 address is divided into two main parts:
  • first 64 bits are used as a network ID
  • the second 64 bits designate a specific interface

• no need for a subnet mask
  • because the network and host portions are fixed size
• network addresses are written using prefix notation
  • where /nn is the length of the routing prefix in bits
  • in the 64-bit network id, the length of any given network prefix is used to determine whether two addresses belong to the same IP network

Example

• most ISPs receive allocations of /32 blocks and issue each customer with a /48 prefix for use on a private network
• A /48 block allows the private network to be configured with up to 65,336 subnets

Global and Link-Local Addressing

• in IPv4, hosts generally have a single IP address per interface
• IPv6 interfaces are more likely to be configured with multiple addresses
  • main types are global and link-local:
    • global address
      • is one that is unique on the Internet (equivalent to public addresses in IPv4)
      • In hex notation, a global address starts with a 2 or with a 3
    • link-local
      • used on the local segment to communicate with neighbor hosts
      • In hex notation, link-local addresses start with fe80::
• most hosts obtain a global and link-local address via the local router
  • process is referred to as StateLess Address Auto Configuration (SLAAC)
• IPv6 hosts do not need to be configured with a default gateway
  • IPv6 uses a protocol called Neighbor Discovery (ND)
    • used to implement SLAAC
    • allows a host to discover a router
    • performs the interface address querying functions performed by ARP in IPv4

Dual Stack

• transitioning from IPv4 has proved enormously difficult
• so, most hosts and routers can operate both IPv4 and IPv6 at the same time
  • referred to as dual stack
• a host will default to attempting to establish an IPv6 connection and fall back to IPv4 if the destination host does not support IPv6