 Hello everyone, I am Mrs. Kanchan Khedikar working as an assistant professor in Computer Science and Engineering department at Valchan Institute of Technology, Solapur. Today we will learn network address translation, it is abbreviated as NAT. Learning outcomes at the end of this session student will be able to discuss NAT technology and they will discuss how it can be used to elevate the shortage in number of addresses in IPv4. Before starting the network address translation in detail, let me explain you what is the need of NAT. We know that ISP who is responsible to distribute the addresses to the network. But the distribution of addresses through ISP has created a problem. ISP has granted a small range of addresses to a small business. But if business grows and need a larger range of addresses then ISP may not able to grant the demand because the addresses before and after the range may have already allotted to the other network. In most of the situation some computers need access to internet. So the number of a located address does not match the number of computers in the network. Assume any small business having a number of PCs. So that particular small business having the 20 number of PCs. Out of these 20 numbers only 5 PCs are connected to the internet. So only 5 addresses are visible. But actually there are 20 addresses. So the technology that can provide the mapping between the private and universal addresses at the same time support the virtual private network is known as network address translation. The technology allows a site to use a set of private addresses for internal communication and set of global internet addresses for communication with the rest of the world. Now let us see in short what actually NAT is. NAT maps private address to public IP address. That means it maps the private IPs to public IP. It is a short term solution to the problem of depletion of IP addresses. It is required because of the shortage of the IPv4 addresses. Now let me explain the network address translation in detail with the help of diagram. The site has only one single connection to the global internet through a NAT capable router that runs the NAT software. In this diagram the NAT software or the NAT router is shown. Now consider this is a private network which has total 20 private addresses. This is a NAT router which is connected to the internet. So here this private address and there is another global address. So NAT router who is responsible to connect private network with global network or we can say that with internet. So NAT router connects one private address and one global address. Now let us see address translation. In this address translation the private network one packet is moved from any one host of the private network to the global network. So here this packet contained source address 172.18.3.1 which is passed through the network address translation router that is NAT router. So NAT router has a software which store the outgoing packet addresses. By transmitting that packet NAT router changes its source address. So once the packet is moved from NAT router it changes its source address. So initially for the outgoing packet the IP address is 172.18.3.1 but whenever that packet is moved from this router it address get changes and it become 200.24.5.8. The same thing is happened with incoming packets. So these two are incoming packets. When this incoming packet is moved to the NAT router, NAT router changes its destination address to the private address. So in this way address translation takes place. Now pause the video and write down the answer. Question is what are the situations where NAT is required? Let us see the answers. When we need to connect to the internet and our host do not have globally unique address then NAT required. When we want to hide the internal IP addresses from outside for security purpose then we require the NAT. And the third one is a company is going to merge in another company which use the same address space then we require the NAT router. Let us move toward the translation table. Why there is a need of translation table? Let us see the translating the source addresses for an outgoing packet it is very straightforward but to know the destination address of incoming packet from internet is difficult because in the internet or in the private network there are so many hosts. So it is very difficult to identify that particular host address. So address translation table is required. So there are two type of address translation table one is by using one IP address only and the another is by using pool of IP addresses. So let us see using one IP address translation table has two columns first column is a private address and another column is the external address router make the note of the destination address. To response come from the or if the response come from the destination then router uses the source address of packet to find the private address. So let us see how it find out the private address. Now using one IP address that means this router allocate only one IP address one global IP address to that packet. So in this diagram this is the private network and this is the outgoing packet which contain source address and destination address. Whenever this packet is reaches to the NAT router NAT router store its addresses that is source address and destination address. So its source address is 172.18.3.1 whereas its destination address is 25.8.2.10. While moving forward this packet to the internet NAT changes its source address and that source address is 200.24.5.8. Now the same thing is happened with the incoming frames. When the incoming packet is arrived at NAT router it check the source address and destination address. So here for this incoming packet source address is 25.8.2.10. So this address is accessed by translation table and this address helps to find out the destination address or the network IP host. So this address for this address the source address is 172.18.3.1 ok. So this address is the destination address. So in this way translation table is needed to find out the incoming as well as outgoing frames or the packets. But there is one disadvantage using only one global address by NAT router it allows only one private network host to access the same external host. So to eliminate this or allow or NAT allow to access the multiple private network we have the solution and that is using a pool of IP addresses. NAT router uses a pool of IP addresses. For example 4 addresses like 200.24.5.8, 5.9, 5.10 and 5.11. So these 4 private network host can communicate with the same external host at the same time. Let us see the advantages of NAT that is network address translation. NAT increases the flexibility when connecting to the internet. It eliminates address renumbering as a network evolves. It conserves legally registered addresses. These are the 3 advantages of NAT. So in this video we have learned NAT technology and the advantages of NAT. This is the reference. Thank you.