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NAT is an address translation technology that works on a router. When an internal network wants to access the Internet, it can be converted into an external network through NAT. The router with NAT is called the NAT router.

Because the current ip address is very scarce, it can not meet the luxury demand of one ip address per person. NAT solves this problem, but it also has its advantages and disadvantages.

There are three ways to implement NAT, namely, static translation Static Nat, dynamic conversion Dynamic Nat and port multiplexing OverLoad.

1 > static translation refers to the translation of private IP addresses in the internal network to public IP addresses. IP address pairs are one-to-one and remain unchanged. A private IP address is only translated into a public IP address. With the help of static conversion, the external network can access some specific devices (such as servers) in the internal network.

2 > dynamic translation means that when the private IP address of the internal network is translated into a public IP address, the IP address is uncertain and random, and all private IP addresses authorized to access the Internet can be randomly translated into any specified legal IP address. That is, dynamic translation can occur as long as you specify which internal addresses can be translated and which legal addresses are used as external addresses. Dynamic translation can use multiple sets of legitimate external addresses. When the legal IP address provided by ISP is slightly less than the number of computers within the network. Dynamic conversion can be used.

Port multiplexing (Port address Translation,PAT) refers to changing the source port of outgoing packets and performing port translation, that is, port address translation (PAT,Port Address Translation). The port multiplexing mode is adopted. All hosts in the internal network can share a legitimate external IP address to access Internet, which can maximize the saving of IP address resources. At the same time, all hosts within the network can be hidden to effectively avoid * from internet. Therefore, port multiplexing is the most widely used in the network at present.

How NAT works is shown in the following figure

Pros and cons of NAT:

Advantages:

1 >: because NAT can be divided into external address and internal address, it has a certain protective effect on the host.

2 >: he solved the current shortage of ip addresses.

3 >: broadband sharing, which is the greatest role of NAT.

Limitations:

(1) NAT violates the design principle of IP address structure model. The IP address structure model is based on the fact that each IP address identifies a network connection. Internet's software design is based on this premise, and NAT makes it possible for many hosts to use the same address, such as 10.0.0.1.

(2) NAT makes IP protocol change from connectionless to connection-oriented. NAT must maintain a mapping of private IP addresses to public IP addresses and port numbers. In TCP/IP protocol system, if a router fails, it will not affect the implementation of TCP protocol. Because it only takes a few seconds to receive no reply, the sending process will enter the timeout retransmission process. When there is NAT, the originally designed TCP/IP protocol process will change, and Internet may become very fragile.

(3) NAT violates the design principle of the basic network hierarchical structure model. Because in the traditional network hierarchical structure model, the N layer can not modify the header content of N + 1 layer. NAT breaks this principle of independence of each layer.

(4) some applications insert IP address into the content of the text, such as standard FTP protocol and IP Phone protocol H.323. If NAT works with this type of protocol, then the NAT protocol must be modified appropriately. At the same time, the transport layer of the network may also use protocols other than TCP and UDP, so NAT must know and modify accordingly. Due to the existence of NAT, it is difficult to implement P2P applications, because P2P file sharing and voice sharing are based on IP protocol.

(5) NAT has influence on high-level protocol and security at the same time. RFC discussed the problems existing in NAT. Opponents of NAT argue that this temporary solution to alleviate the shortage of IP addresses delays the process of Ipv6 migration and does not solve the deep-seated problems, which they consider undesirable.

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