 Now let's look at this figure. In the center you have this box. This box actually represents a router. What does a router do? A router is an endpoint of a network or in other words it connects two different networks to each other. A router may have multiple interfaces but on each interface it is a separate network. So you can think of a router as a post office where the packets from one area come and then they are looked up and sent to other areas along different routes. So let's look at one side of this network. So on one side you see addresses of the form 10.1.1.2 and so on. At the bottom we see that an IP address is nothing but a 32 bit number. So it's a 32 bit binary number which for ease of human comprehension is split into four octets. So each octet is now read in the corresponding decimal form. So that's how one side of the network becomes what is called the 10.1.1 network. So the network is often referred to using the name that is given to that router's interface. So 10.1.1.1 is the router which is catering to the entire network of the other IP addresses that we see. So each of these can be machines or they can even be local networks inside that. We will see how that happens a bit later. So what this router is doing is it's connecting the 10.1.1 network to the 223.1 network. So what does a router have? Suppose you look at the machine which says okay 10.1.1.2 wants to send a packet to the server which is at 223.1.5.2. So essentially what will happen is that the machine which is there at one end of the network, it knows where to send the packet in order to reach the router. In the router there will be a table which will say that okay if a packet comes on this line with this IP address and wants to go out to that IP address then it has to be sent out on that line. So this is a very conceptual level of explanation. Essentially the routing table is simply mapping the input link, the input IP address to the corresponding output IP address and the output link. So the router knows which is the link on which to send the packet or forward the packet so that it goes towards the 223.1.5.2 machine. While this is a very simplistic example, we have only considered one router and two interfaces. The same idea keeps on extending in order for us to reach from anywhere in the internet to anywhere else in the internet.