Network Virtualization
Network virtualization is a technology which enables combining computer network resources into a single platform appearing as a single network. In this form of virtualization all hardware and software in the virtual network appear as a single collection of resource. There is another form of network virtualization in which logically isolated network partitions are created over the shared physical network infrastructure.
Figure: Logical Network partitions on a shared network infrastructure
Such logical partitioning, as shown in the figure, resulting in multiple heterogeneous virtual networks can coexist simultaneously over the shared infrastructure.
网络虚拟化是在一个已有的网络上构建多个不同的虚拟网络。，每个虚拟网络被设计成支持一个特定的分布式应用。例如， 一个虚拟化网络可以支持多媒体流，它和支持多人在线游戏的虚拟网络共存；他们都在相同的底层网络上运行。每个虚拟网络有它自己特定的寻址模式、协议和路由算法，他们被重新定义来满足特定类应用的需求。
Overlay Network
Overlay networking (aka SDN overlay) is a method of using software to create layers of network abstraction that can be used to run multiple separate, discrete virtualized network layers on top of the physical network, often providing new applications or security benefits.
Nodes in the overlay network can be thought of as being connected by virtual or logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network. For example, distributed systems such as peer-to-peer networks and client-server applications are overlay networks because their nodes run on top of the Internet.
The Internet was originally built as an overlay upon the telephone network, while today (through the advent of VoIP), the telephone network is increasingly turning into an overlay network built on top of the Internet.
覆盖网络（Overlay Network）是一个节点和虚拟链接组成的虚拟网络，它位于一个底层网络（例如IP网络）之上，进而提供一些独有的功能：
- 满足一类应用需求的服务或一个特别高层的服务，例如，多媒体内容的分发。
- 在一个给定的联网环境中更有效的操作，例如，在一个组织网络中的路由。
- 额外的特色，例如，组播或安全通讯。
覆盖网增加网络服务的复杂性，但是在未改变底层网络的前提下增加了其自由度。
覆盖网络(overlay network)简单说来覆盖网络就是应用层网络，它是面向应用层的，不考虑或很少考虑网络层，物理层的问题。覆盖网络允许对没有IP地址标识的目的主机路由信息，例如：Freenet和DHT（分布式哈希表）可以路由信息到一个存储特定文件的结点，而这个结点的IP地址事先并不知道。比如说,P2P就是一种覆盖网络P2P是集中分散分形概念的同行对等覆盖网络，网络覆盖就是指用户能够接收到网络信号的范围。比如说网络覆盖10米半径，就是指以发射点为圆心10米半径内的设备都可以收到信号。通常指无线信号网络，如手机网络，无线网络等设备的有效信号接收范围
分布式系统：概念与设计 第五版
White Paper on “NETWORK VIRTUALIZATION”
http://tec.gov.in/pdf/Studypaper/White%20Paper%20on%20NetworkVirtualization.pdf
What is Overlay Networking (SDN Overlay)?
https://www.sdxcentral.com/networking/sdn/definitions/what-is-overlay-networking/
Overlay network
https://en.wikipedia.org/wiki/Overlay_network#List_of_overlay_network_protocols
覆盖网络
https://baike.baidu.com/item/%E8%A6%86%E7%9B%96%E7%BD%91%E7%BB%9C
