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Global Unicast Address in CCNA

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Global Unicast Address is equivalent to IPv4 public address. Global Unicast Addresses in IPv6 are globally identifiable and uniquely addressable.

For more details, you can refer to IPv4 Addressing and IPv6 Addressing articles.

Global Unicast Address Format


The Most significant 48-bits are designated as global routing prefix which is assigned to a specific automatic system. The three most significant bits of the global routing prefix are always set to 001.

Global Unicast Address (GUA):

  • 2000::/3 (First hextet: 2000::/3 to 3FFF::/3).
  • Globally unique and routable.
  • Similar to public IPv4 addresses.
  • 2001:db8::/32 – RFC 2839 and RFC 6890 reserve this range of addresses for documentation.

Structure of Global Unicast Address in IP v4 and IP v6


IPv4 and IPv6 in Global Unicast Address


  • 64-bit Interface ID = 18 quintillion (18,446,744,073,709,551,616) devices/subnet.
  • 16 bit subnet ID (initially recommended) = 65,536 subnets

IPv6 Global Unicast Address Format Fields:

1. Global routing prefix: global routing prefix is the portion of the address that is assigned by the provider such as an ISP to a customer or site. The most significant 48-bits are assigned as a Global routing prefix which is assigned to a specific autonomous system.

2. Subnet ID: The subnet ID is the portion between the global routing prefix and the interface ID.

3. Interface ID: The Interface ID is equal to the host part of an IPv4 address, It is must recommend that in most cases /64 subnets must be used which creates a 64-bit ID.

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Last Updated : 24 Jun, 2022
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