IP第6个版本（IPv6），是互联网协议的新版本，设计为IP第4版本（IPv4，RFC-791）的继任。从IPv4升级到IPv6主要的改变有以下几类：
- 扩展地址容量
IPv6将IP地址的位址从32位提升到128位，支持更多级别的地址继承，极大量的地址节点，更简单的自动地址分配。通过在多播地址中添加的一个“SCOPE”域，多播路由的可扩展性获得提高。引入了一个新的地址类型，叫做任意地址（anycast address），用于将包发送给一组节点中的任意一个地址。
- IP头简化
一些IPv4下头部内容已经被削减或者变成可选的了，这是为了减少常用处理的包和限制IPv6头所占用的带宽。
- 更好地支持扩展和选项功能
- 流量标签功能
- 认证和隐私功能
原文如下：
IP version 6 (IPv6) is a new version of the Internet Protocol, designed as the successor to IP version 4 (IPv4) [RFC-791]. The changes from IPv4 to IPv6 fall primarily into the following categories:
- Expanded Addressing Capabilities
IPv6 increases the IP address size from 32 bits to 128 bits, to support more levels of addressing hierarchy, a much greater number of addressable nodes, and simpler auto-configuration of addresses. The scalability of multicast routing is improved by adding a "scope" field to multicast addresses. And a new type of address called an "anycast address" is defined, used to send a packet to any one of a group of nodes.
- Header Format Simplification
Some IPv4 header fields have been dropped or made optional, to reduce the common-case processing cost of packet handling and to limit the bandwidth cost of the IPv6 header.
- Improved Support for Extensions and Options
Changes in the way IP header options are encoded allows for more efficient forwarding, less stringent limits on the length of options, and greater flexibility for introducing new options in the future.
- Flow Labeling Capability
A new capability is added to enable the labeling of packets belonging to particular traffic "flows" for which the sender requests special handling, such as non-default quality of service or "real-time" service.
- Authentication and Privacy Capabilities
Extensions to support authentication, data integrity, and (optional) data confidentiality are specified for IPv6.