Network Protocol 03 - Routing and NAT

Routing

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As previously known, forwarding data between different network segments requires the support of routers.However, by default, the router only knows the network segment directly connected to it, so the network segment that is not directly connected needs to be told to it through static routing and dynamic routing.

• Static routing: administrators manually add routing information, suitable for small-scale networks
• Dynamic routing: routers automatically obtain routing information through routing protocols (such as RIP, OSPF), suitable for large-scale networks

First of all, let's talk about how to manually add routing information for static routing. As shown in the figure below, how to configure four hosts to communicate with each other?

In the above picture, computers 0 and 1 can communicate with each other, Router 0 knows each other's network segments, and the same is true for computers 2 and 3, but it seems that computer 0 can't communicate with computer 3.Here, the concept of routing table needs to be introduced. By adding the static route of router 0, it can access computers that are not directly connected to the network segment. Of course, the configuration method of router 1 is the same.

In this static route, via network and subThe network mask sets the specific IP or network segment of the computer that needs to be reached, and the next hop is to set the IP of other routers connected to router 0 (in the same network segment, so the default subnet mask is the same), indicating that the data is forwarded by this router..There are several ways to set the routing table of router 0

• Host-specific routing
  
  thisThe method is to directly determine the IP of the target computer, and the range is the IP of a specific computer

• Network routing
  
  this waySet the network segment where the target computing is located. If switch 3 is connected to computer 4 in the above figure, it can also be reached. The range is the upper right or lower right network segment

• Summary route
  
  
  This method enables all computers in the above network segment to receive data,The range is the entire network segment on the right (provided that all computers meet the above network segment)

• Default route
  
  this wayThat is, when the router does not know how to go, it will choose to take the default route, that is, the next hop (If there is a designated route, then the designated route is preferred), and the range is larger than the above summary.The route is wider, as long as all the network segments connected to the next hop router can go

NAT (Network Address Translation)

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Before explaining NAT, you first need to know the public IP and private IP

• Public network IP: The routers on the Internet only have the routing table for reaching the public network, but not the routing table for reaching the private network. The public network IP is uniformly allocated and managed by the Internet Network Information Center (Internet NIC), and the ISP(Internet service providers, such as mobile, telecommunications, etc.) need to apply for a public network IP from the Internet NIC
• Private network IP: mainly used for LAN, the following are reserved private network segments
  • Class A: 10.0.0.0/8, 1 Class A network
  • Class B: 172.16.0.0/16 ~ 172.31.0.0/16, 16 Class B networks
  • 192.168.0.0/24 ~ 192.168.255.0/24, 256 Class C networks

General home networks are private IPs. When we want to access the Internet (public IP), we need to perform NAT translation, which will convert the private IP to the public IP. This step can be done by the router.For example, search the local IP address on a search engine, which is the converted public IP address (not necessarily the IP address of the router at home, because it may be that many routers have undergone multiple NAT translations)

The feature of NAT is that it hides the internal real IP and saves public IP resources at the same time.NAT is divided into the following ways:

• Static conversion: Manually configure the NAT mapping table to convert public IP and private IP one-to-one. This method cannot achieve the purpose of saving public IP and is not commonly used
• Dynamic conversion: Define an external IP address pool, and dynamically and randomly convert public network IP and private network IP one-to-one, still can not achieve the purpose of saving public network IP, not commonly used
• PAT (Port Address Translation): It can perform many-to-one conversion between multiple private network IPs and one public network IP, saving public network IP resources to the greatest extent.Using port multiplexing, each private network IP corresponds to a unique port number, and different data streams are identified by the port number.This is currently the most widely used NAT implementation.