Before explaining EUI-64, let us take a look at IPv6 (Internet Protocol version 6). IPv6 was developed to address the limitations of the previous version, IPv4, which had a limited address space and was running out of available addresses. Even though there are several solutions that have been developed to help preserve IPv4 addresses, such as NAT, PAT, and CIDR, the adoption of IPv6 is still necessary for the long-term sustainability of the Internet. IPv6 uses 128-bit addresses, which provides a much larger address space than IPv4 and enables more devices to be connected to the Internet. In IPv6, the first 64 bits of the address are used to represent the network prefix whereas the last 64 bits are to represent the host portion.

EUI-64 (Extended Unique Identifier) is a technique utilized to enable automatic configuration of IPv6 host addresses. When generating a unique 64-bit interface ID, an IPv6 device will utilize the MAC address of its interface. Nevertheless, since the MAC address is only 48 bits in length, there is a shortfall of bits for the interface ID. The question is, how can we address this shortage in the number of bits?

For instance, if the MAC address is 1234.5678.9ABC, then the 7 ^th bit is in the second hexadecimal character which is (2)₁₆ so (0010)₂ will become (0000)₂ which is (0)₁₆ in hexadecimal. This is what the interface ID will become:

There are different types of IPv6 addresses, each serving a specific purpose. Let us take a look at each type and its uses in the following list;

• Global Unicast
  • Similar to IPv4 public addresses, routable over the internet
  • Defined in the 2000::/3 block
• Unique Local
  • Similar to IPv4 private addresses, cannot be used over the internet
  • Do not have be to be gloabally unique
  • Defined in the FC00::/7 block
• Link-local
  • Automatically generated using eui-64
  • Used for communication over a single link or network segment
  • Not routable
  • Defined in the FE80::/10 block
    • 64 bits to define the network portion
    • 54 bits after FE80/10 are all zeors
    • 64 bits to define the host portion using eui-64
• Multicast Addresses
  • IPv6 is not capable of broadcast
  • Defined in FF00::/8 block
• Anycast Addresses
  • New feature in IPv6
  • One-to-one of many or one-to-nearest communication
  • Created by assigning the same IPv6 address to multiple interfaces or devices in different locations
• The unspecified address
  • Used to inform routers about the default route
  • Defined in ::/0
• The loopback address
  • used to test the NIC card and TCP/IP stack on a device
  • Defined in ::1