To access the Internet, one public IP address is needed, but we can use a private IP address in our private network. The idea of NAT is to allow multiple devices to lớn access the mạng internet through a single public address. To achieve this, the translation of a private IP address khổng lồ a public IP address is required. Network Address Translation (NAT) is a process in which one or more local IP address is translated into one or more Global IP address and vice versa in order to lớn provide mạng internet access to lớn the local hosts. Also, it does the translation of port numbers i.e. Masks the port number of the host with another port number, in the packet that will be routed khổng lồ the destination. It then makes the corresponding entries of IP address và port number in the NAT table. NAT generally operates on a router or firewall.

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Network Address Translation (NAT) working –Generally, the border router is configured for NAT i.e the router which has one interface in the local (inside) network & one interface in the global (outside) network. When a packet traverse outside the local (inside) network, then NAT converts that local (private) IP address to a global (public) IP address. When a packet enters the local network, the global (public) IP address is converted khổng lồ a local (private) IP address.

If NAT runs out of addresses, i.e., no address is left in the pool configured then the packets will be dropped và an mạng internet Control Message Protocol (ICMP) host unreachable packet to the destination is sent.

Why mask port numbers ? Suppose, in a network, two hosts A & B are connected. Now, both of them request for the same destination, on the same port number, say 1000, on the host side, at the same time. If NAT does only translation of IP addresses, then when their packets will arrive at the NAT, both of their IP addresses would be masked by the public IP address of the network và sent khổng lồ the destination. Destination will send replies lớn the public IP address of the router. Thus, on receiving a reply, it will be unclear lớn NAT as khổng lồ which reply belongs khổng lồ which host (because source port numbers for both A and B are the same). Hence, to lớn avoid such a problem, NAT masks the source port number as well and makes an entry in the NAT table.

NAT inside and outside addresses –Inside refers to the addresses which must be translated. Outside refers khổng lồ the addresses which are not in control of an organization. These are the network Addresses in which the translation of the addresses will be done.

Inside local address – An IP address that is assigned khổng lồ a host on the Inside (local) network. The address is probably not an IP address assigned by the service provider i.e., these are private IP addresses. This is the inside host seen from the inside network.Inside global address – IP address that represents one or more inside local IP addresses to lớn the outside world. This is the inside host as seen from the outside network.Outside local address – This is the actual IP address of the destination host in the local network after translation.

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Outside global address – This is the outside host as seen from the outside network. It is the IP address of the outside destination host before translation.

Network Address Translation (NAT) Types –There are 3 ways khổng lồ configure NAT:

Static NAT – In this, a single unregistered (Private) IP address is mapped with a legally registered (Public) IP address i.e one-to-one mapping between local and global addresses. This is generally used for website hosting. These are not used in organizations as there are many devices that will need mạng internet access và to provide mạng internet access, a public IP address is needed.

Suppose, if there are 3000 devices that need access to lớn the Internet, the organization has lớn buy 3000 public addresses that will be very costly.Dynamic NAT – In this type of NAT, an unregistered IP address is translated into a registered (Public) IP address from a pool of public IP addresses. If the IP address of the pool is not free, then the packet will be dropped as only a fixed number of private IP addresses can be translated to lớn public addresses.Suppose, if there is a pool of 2 public IP addresses then only 2 private IP addresses can be translated at a given time. If 3rd private IP address wants to lớn access the mạng internet then the packet will be dropped therefore many private IP addresses are mapped khổng lồ a pool of public IP addresses. NAT is used when the number of users who want to lớn access the internet is fixed. This is also very costly as the organization has lớn buy many global IP addresses khổng lồ make a pool.Port Address Translation (PAT) – This is also known as NAT overload. In this, many local (private) IP addresses can be translated lớn a single registered IP address. Port numbers are used to distinguish the traffic i.e., which traffic belongs khổng lồ which IP address. This is most frequently used as it is cost-effective as thousands of users can be connected to lớn the mạng internet by using only one real global (public) IP address.Advantages of NAT –

NAT conserves legally registered IP addresses.It provides privacy as the device’s IP address, sending and receiving the traffic, will be hidden.Eliminates address renumbering when a network evolves.

Disadvantage of NAT –

Translation results in switching path delays.Certain applications will not function while NAT is enabled.Complicates tunneling protocols such as IPsec.Also, the router being a network layer device, should not tamper with port numbers(transport layer) but it has to vày so because of NAT.