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IPv6’s address structure is defined by the IETF as part of RFC 3513 and provides many of the previously discussed advantages over IPv4. At first glance, it is evident an IPv6 address looks nothing like an IPv4 address. IPv4 addresses are composed of four numerical octets, ranging from 0 to 255, separated by periods, forming a 32-bit address. IPv6 addresses are 128 bits and consisting of a series of eight segments, separated by a colon. Each segment is a 4-character hexadecimal number, ranging from 0000 to FFFF, representing 16 bits each, totaling to the 128 bits.
For display purposes, an IPv6 address may be presented without leading zeros. For example, an IPv6 address of 2001 : 0DC8: E004 : 0001 : 0000 : 0000 : 0000 : F00A could be displayed as 2001 : DC8: E004 : 1 : 0 : 0 : 0 : F00A.
The address may be further reduced by removing consecutive fields of zeros and replacing with a double- colon. The double-colon can only be used once in an address. The address becomes 2001 : DC8 : E004 : 1 :: F00A.
IPv6 offers the following address types:
An IPv6 Unicast address is composed of the Global Routing Prefix, Subnet ID, and the Interface Identifier. The Global Routing Prefix is the network ID or prefix of the address for routing. The Subnet ID is similar to the netmask in IPv4 but is not part of the IP address in IPv6. Finally, the Interface ID is a unique identifier for a particular interface. For Ethernet networks, the Ethernet MAC address (48 bits) may be used for the Interface Identifier, by inserting 16 additional bits, forming what is referred to as an EUI-64 address.