Virtual Internet

Virtual Internet

Networking equipment

Intermediate device , also known as intermediate systems or relay (Relay) system .

• The physical layer repeater system: a repeater (repeater), as will be appreciated connector (the Hub);
• The data link layer relay system: a bridge or bridge (Bridge);
• Network layer relay system: a router (Router);
• Above network layer relay system: a gateway (Gateway); the gateway is to another network address of the router interface .

Networking equipment: routers

• When the relay system is a repeater or a bridge, the interconnection network is generally not referred to, this is only because the expansion of a network, but this is still a network;
• As the gateway relatively complex, less frequently used;
• It refers to the Internet network by interconnecting the routers;
• Due to historical reasons, many documents related to a TCP / IP router network layer is called the gateway .

The figure above, three computers you want to access other networks such as the Internet must pass through the gateway Router1; but you can access the local network segment. The computer may be provided while a plurality of gateways, as shown in FIG.

As shown, PC2 Internet through gateway Router1; may be the Internet through gateway Router4.

Summary : the gateway like a door, computer data must want to go out through the door; a computer can have multiple doors.

A network gateway is generally disposed on the router connected to the network, the port address is a router; the computer network router to find and access the network through the gateway router (address)

梳理一下PC1上网的过程，上网的实质就是同网段或不同网段内双方的通信。当需要访问的互联网与计算机在同一网段内，可以不经路由器中转直接通信，比如PC1可以不经路由器直接访问PC0；否则需要经过路由器中转才能访问其他网段的互联网，比如PC1想要访问其他网段的Cloud0 。

而判断通信双方是否在同一网段根据的是网关。某网段的网关一般设置在连接该网段的路由器上，是路由器上一个端口的IP地址，该地址属于路由器连接的网段。如图中PC1的IP地址为192.0.1.5与路由器的网关192.0.1.1同属网段192.0.1；其他的网段也是相同的原理，于是可以通过之前提到过的子网掩码求与运算比较连接各网段的路由器上的网关是否在同一网段。

所以，图中PC1需要访问Cloud0时，首先通过网关判断出了通信双方处于不同网段，随后PC1把数据报通过网关这个地址发送给Router0，路由器Router0通过查询内部路径表寻找传输的最佳路径，当中转到路由器Router1时，Router1发现数据报中的目标IP地址与自己的网关同属一个网段，最后将数据报传输给该网段下的Cloud0完成通信。这就是访问网络的过程。

互联网的问题

互联在一起的网络要进行通信，会遇到许多问题需要解决，如：

• 不同的寻址方案；
• 不同的最大分组长度；
• 不同的网络接入机制；
• 不同的超时控制；
• 不同的差错恢复方法；
• 不同的状态报告方法；
• 不同的路由选择技术；
• 不同的用户接入控制；
• 不同的服务（面向连接服务和无连接服务）；
• 不同的管理与控制方式 ；

分组在互联网中的传送

这里我们只考虑，各设备的网络层如何在各设备中中转从而到达目的地址。

互联网络与虚拟互联网络

• 互联网络：里面充满了各种网络设备，有着不同的技术。
• Virtual Internet: computer to access the Internet when another computer, regardless of other devices on the network, the entire Internet equivalent of a large network is virtual, it shields the complexity of the network, such as network, each server has IP address, direct access to the IP address of the computer on the list. This is the virtual network, we only need to know the IP address of the access on it, you do not care about how the internal network connection.

Meaning virtual interconnection network

• The so-called virtual interconnection network is a logical interconnection network, which means that a variety of heterogeneous physical network interconnecting could have been an objective reality, but we can make use of IP protocols these different properties from the user network looks like a unified network.
• IP protocol virtual interconnection network can be referred to as the IP network.
• The benefits of using virtual interconnection network is: When the host on the Internet to communicate, communicate as if on a network, and each specific heterogeneous network interconnection unseen details.

IP protocol Introduction

Internet Protocol (IP) is one of the TCP / IP system in two major agreements. There are four supporting :( protocol used in the IP protocol only four network layer protocol used )

• ARP ARP
  (Address Resolution Protocol)
• Reverse Address Resolution Protocol RARP
  (Reverse Address Resolution Protocol)
• Internet Control Message Protocol ICMP
  (Internet Control the Message Protocol)
• Internet Group Management Protocol IGMP
  (Internet Group Management Protocol)

Level between the network layer protocol 4

FIG seen, four network layer protocol, ARP protocol more underlying, IP protocol dependent on the ARP protocol. ICMP protocol relies on the IP protocol; the IGMP protocol relies on the ICMP protocol.