Learning IPv6 - Important Points

  • Its a 128bit addresses, divided into 8 groups of 4 Hexadecimal characters each.
  •  128 bits IPv6 address is divided in to 64 bit Network and 64 bit Host portion.  The 64 bit Network portion is further divided into Global Routing Prefix (48bits) and Subnet (16 bits) fields. The Enterprises or Providers are assigned /48 addresses and have 16 bits of subnetting available.
  • The Global Routing Prefix (48 bits) have following elements:
           The first three bits (/3) of a unicast address are always 001.
           The next 13 bits (/16) identify the Top-Level Aggregator (TLA); the upstream ISP.
           The next 24 bits (/40) identify the next-level aggregator, or regional ISP.       
  • Rules to define an IPv6 address:
         1. Eliminate groups of consecutive zeros once per address.


         2. Drop leading zeros.

  • There are two Types of IPv6 Address.   Link Local Address and  Global Address. Each interface will be assigned one Link Local and a Global address.
  • A Link-Local address starts with FE80.  Below are details to define a Link Local address.
  • FE80 + Followed by 54 zeros + 48 bit mac-address of interface (with some difference)  inserted with FFFE in the middle.
                   R1# sh int f0/0 | i bia
                   Hardware is DEC21140, address is ca01.2f2c.0000 (bia ca01.2f2c.0000)
                   R1#sh ipv6 int f0/0
                   FastEthernet0/0 is up, line protocol is up
                   IPv6 is enabled, link-local address is FE80::C801:2FFF:FE2C:0
                   No Virtual link-local address(es):
                   Global unicast address(es):
                   2001:1111:A::1, subnet is 2001:1111:A::/64
  • A Link-Local address enables a node to communicate with other nodes on the link.
  • A Link-Local addresses are not routable in the network.
  • Global addresses have high level 3 bits to 001 (2000::/3).
  • IPv6 only support Unicast and Multicast type of traffic and no Broadcast exists in IPv6. Protocols like ARP/Gratuitous ARP are not used in IPv6. IPv6 uses Neighbor Discovery mechanism to identify duplicate addresses assigned on link. Alternate command for "show ip arp" in IPv6 is "show ipv6 neighbors"
            R1#sho ipv6 neighbors
            IPv6 Address                               Age   Link-layer Addr   State        Interface
            2001:1111:A::2                              9      ca02.073c.0000   STALE       Fa0/0
            FE80::C802:7FF:FE3C:0              9      ca02.073c.0000   STALE       Fa0/0
  • No NAT or PAT is required for IPv6 addressing.
  • There are tunneling techniques  that we can use to transport ipv6 over ipv4 network to support Backward compatibility.
  • Running IPv4 and IPv6 simultaneously is called “Dual Stack”.
  • IPv6 unicast-routing and ipv6 cef are not enabled by default and must be configured separately.  "ipv6 unicast-routing" command is necessary if you want to run any IPv6 IGP routing protocol. For example RIPng, OSPFv3 etc.
          R1(config)#ipv6 unicast-routing
          R1(config)#ipv6 cef  
  • IPv6 uses "Stateless Autoconfiguration" method to automatically assign IPv6 addresses to hosts or interfaces on the node similar to DHCP.
  • DNS service is also available for IPv6.  Like A- Resource Record is used for hostname to IP resolution, AAAA Resource Record is used for hostname to IPv6 address resolution. See my blog "How DNS Works?"
  • You can manually assign/configure more than one IPv6 address on an interface of node.
            R1#sh run int f0/0
             interface FastEthernet0/0
             no ip address
             duplex full
             ipv6 address 2001:1111:A::1/64
             ipv6 address 2001:2222:B::1/64

  • Multicast address in IPv6 start from FF0x. For example Multicast addresses  for "All-Hosts" and "All-Routers" are  is FF02::1 and FF02::2 respectively.
  • For IPv6 running and Network Layer, Ethernet -Type field value for an Ethernet Frame is 0x86DD
  • As we have range reserved as Loopback address range in IPv4, Loopback IP reserved for IPv6 is 0000:0000:0000:0000:0000:0000:0000:0001/128 or ::1/128
  • Default route in IPv6 is defined as (::/0)