Data Communication Computer Network 简明教程

Network Layer Introduction

What is Network Layer (Layer-3)?

OSI 模型中的第 3 层称为网络层。网络层负责管理与主机和网络寻址、管理子网络和互连网络有关的选项。

Layer-3 in the OSI model is called Network layer. Network layer manages options pertaining to host and network addressing, managing sub-networks, and internetworking.

网络层负责在子网内或子网外将数据包从源路由到目标。两个不同的子网可能具有不同的寻址方案或不兼容的寻址类型。与协议一样，两个不同的子网可能运行不同的协议，而这些协议彼此不兼容。网络层负责将数据包从源路由到目标，映射不同的寻址方案和协议。

Network layer takes the responsibility for routing packets from source to destination within or outside a subnet. Two different subnet may have different addressing schemes or non-compatible addressing types. Same with protocols, two different subnet may be operating on different protocols which are not compatible with each other. Network layer has the responsibility to route the packets from source to destination, mapping different addressing schemes and protocols.

Network Layer (Layer-3) Functionalities

在网络层工作的设备主要专注于路由。路由可能包括旨在实现单个目标的各种任务。这些任务可以是：

Devices which work on Network Layer mainly focus on routing. Routing may include various tasks aimed to achieve a single goal. These can be:

  1. Addressing devices and networks.

  2. Populating routing tables or static routes.

  3. Queuing incoming and outgoing data and then forwarding them according to quality of service constraints set for those packets.

  4. Internetworking between two different subnets.

  5. Delivering packets to destination with best efforts.

  6. Provides connection oriented and connection less mechanism.

Network Layer Features

第 3 层凭借其标准功能可提供各种特性，如：

With its standard functionalities, Layer 3 can provide various features as:

  1. Quality of service management

  2. Load balancing and link management

  3. Security

  4. Interrelation of different protocols and subnets with different schema.

  5. Different logical network design over the physical network design.

  6. L3 VPN and tunnels can be used to provide end to end dedicated connectivity.

互联网协议是广受尊重和部署的网络层协议，它有助于在互联网上实现端到端设备通信。它有两种类型。IPv4 已统治世界数十年，但现在地址空间即将耗尽。IPv6 被创建出来以取代 IPv4，并有望缓解 IPv4 的局限性。

Internet protocol is widely respected and deployed Network Layer protocol which helps to communicate end to end devices over the internet. It comes in two flavors. IPv4 which has ruled the world for decades but now is running out of address space. IPv6 is created to replace IPv4 and hopefully mitigates limitations of IPv4 too.