The length of an IPv6 address is 128 bits.

IPv6, which stands for Internet Protocol version 6, is the most recent version of the Internet Protocol. It was introduced to address the limitations of its predecessor, IPv4, which uses 32-bit addresses and has a limited number of available addresses.

IPv6 Address Format:
IPv6 addresses are represented as eight groups of four hexadecimal digits, separated by colons. Each group represents 16 bits, and thus the total length of an IPv6 address is 8 groups * 16 bits/group = 128 bits. This allows for a significantly larger number of unique addresses compared to IPv4.

Expanded Notation:
IPv6 addresses can be expressed in compressed or expanded notation. In compressed notation, consecutive groups of zeros can be replaced with a double colon (::), but this can only be done once in an address. For example, the address "2001:0db8:0000:0000:0000:0000:1428:57ab" can be compressed to "2001:db8::1428:57ab".

Benefits of IPv6:
1. Address Space: IPv6 provides a vastly larger address space, allowing for trillions of unique addresses. This ensures that there are enough addresses for all devices and services connected to the Internet.
2. Auto-Configuration: IPv6 supports efficient auto-configuration of network settings, making it easier for devices to connect to a network without manual configuration.
3. Simplified Routing: IPv6 includes features that simplify routing and improve network efficiency.
4. Security Enhancements: IPv6 includes built-in support for IPsec (Internet Protocol Security), which provides encryption and authentication for network communications.

Transition from IPv4 to IPv6:
As the adoption of IPv6 continues to grow, there is a need for a transition from IPv4 to IPv6. This involves coexistence mechanisms such as dual-stack (supporting both IPv4 and IPv6), tunneling (encapsulating IPv6 packets within IPv4 packets), and translation (converting IPv6 packets to IPv4 packets and vice versa).

In conclusion, the length of an IPv6 address is 128 bits, which provides a significantly larger address space compared to IPv4. IPv6 offers numerous benefits and is essential for the continued growth of the Internet.