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NAT

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Introduction

The Network Address Translation (NAT) protocol is mainly used for mutual conversion between Wide Area Network (WAN) IP addresses and Local Area Network (LAN) IP addresses.

Details

A home router has a WAN IP address, and all other devices connected to the router share this WAN IP address. At this time, the NAT protocol needs to be used for conversion.

Private Network Segments

Network TypeRange
Class A10.0.0.0～10.255.255.255
Class B172.16.0.0～172.31.255.255
Class C192.168.0.0～192.168.255.255

These three network segments are private network segments, used for internal LAN use.

Working Principle

As shown in the above diagram, when Host A sends a packet to the server, the router will convert the source IP 192.168.50.2 to 113.112.111.110, and then send it to the server 133.133.133.133 on port 80. When the server returns data, it needs to convert the target address back to the intranet address.

Conversion Process

  • When Host A sends a request packet, a random port number such as 8888 is assigned to the request, that is, the request source is 192.168.50.2:8888
  • When the request reaches the router, the router will also assign a port to the request, such as 9999, and change the source IP address of the request to 113.112.111.110, the final request source address is 113.112.111.110:9999, the router will save this mapping to the NAT conversion table: 192.168.50.2:8888 -- 113.112.111.110:9999.
  • The target address of the data packet returned by the server is 113.112.111.110:9999. When the data packet reaches the router, the router will convert the request target address to 192.168.50.2:8888 according to the records in the conversion table and send the data packet to the intranet host.

Advantages of NAT

  • Network Security: The NAT protocol shares a unified IP address for all network devices in the intranet, preventing intranet hosts from being directly exposed to the public network, which can enhance the security of intranet hosts.
  • Load Balancing: The NAT protocol can redirect external requests to different hosts internally, achieving load balancing.
  • Save IP Addresses: We know that IPV4 addresses are limited, and the NAT protocol allows a large number of intranet devices to connect to the internet, greatly alleviating the problem caused by the shortage of IPV4 addresses.

Disadvantages of NAT

  • Internal Shielding: External servers cannot directly access intranet devices
  • Performance Loss: Router maintenance of the NAT conversion table has performance loss
  • Link Loss: If the router restarts, all TCP connections will be disconnected

Solution

The emergence of the IPV6 protocol can make up for the shortcomings of NAT and is the main application in the future.