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Couple of things to talk about in this module here, one are the IP addressing rules and the other is logical AND operation. The IP address rules are important to talk about because it helps identify potential problems, it helps you troubleshoot IP addresses and will also help you configure your networks so they best take advantage of the available IPs. Here are some of the rules with which you just have to be familiar, you have to have these locked into your noggin before you take the test. One is the network ID can't be set to 127, a lot of these by the way we touched on earlier on I just want to tie them together here. The 127 address is reversed for loop back or that whole network, for loop back and diagnostic purposes, for example to ping your local computer to see if TCP/IP is working on your computer. You can ping the 127.0.0.1 and that is what's known as the loop back address, you can ping your own computer using that address. So there's no computers on the Internet who live on the 127 network. The other rules network IDs and host IDs can't be set to all ones, they can't be set to all zeros. If all bits are set to 1, the address is interpreted as a broadcast address and it's not routed to other networks under normal circumstances. In other words a router doesn't pass broadcast traffic unless it's specifically been configured to do so and all zeros rule is if all the host bits are set to zero for example, that's the same as the network number. To demonstrate just very briefly what I mean here, come look at this document here and just have the IP address that you might have 192.168.2. if I set the IP address to zero and I'm using a subnet mask of 255.255.255.0. If I set the IP address, the host portion to all zeros and again look at this the way the computer sees it. The host portion would be all zeros. That's the same as the network identifier, in other words the 192.168.2.0 network. So can't have that If we set the host portion to all ones, that is interpreted as a broadcast to the entire 192.168.2 network. So those are why those two rules are in effect, those are why in the previous module I subtracted two from the available number of host IDs when I use that 2 to the power of type shortcut. Also unique network IDs are needed for all networks that are connected to the public Internet. Because all networks are connected to each other, each one must be unique. It's the same way that the publicly switch telephone network works. If we want to add another area code we must make sure first that that area code is not in effect. There's already a 913 area code there's already a 913 network of phones, so if we add another if we divide L.A. up again to another area code, it can't be the 913 area code because that would route calls to the Kansas City area, at least the Kansas side any way. Another rule, every host must be unique to the local network ID, again think of the phone analogy. If we have the 913 area code, then we can't have two phone numbers within that area code that are the same. Within the 192.168.100 network there can't be two host IDs of six. Finally every TCP/IP host requires a subnet mask. Why is it required? Because without the subnet mask the IP address itself is meaningless. To demonstrate what I mean here another example that I use in class quite a bit, this helps reinforce the concept of the subnet mask, if we have two computers that have this IP address, is this the same computer or different computers? Well the answer is, we don't know or depends, like the answer to every computer question. We don't have enough information yet because we have not seen the subnet mask. So, now we type in the subnet mask, 255.255.255.0, now are these two IP addresses the same computer or different computers? They're different. This computer is the 2.100 host on the 192.168 network. This computer is the 100 host on the 192.168.2 network. Two separate networks, two separate computers. So these are why these rules are in effect, these are crucial to have in mind as you study IP addressing. The other thing we talk about in this module is the logical AND operator. This is the computer's way of evaluating the destination of an IP packet. Destination is either going to be sent to the local network or sent to the remote network via the default gateway. Here's how the logical AND operator works. It compares the source IP address and the destination IP address and where one and ones occur, the AND operator results in a one. Where zero occurs, it results in a zero. Let's take a look at what this might look like, if we were to take a look inside an IP packet, here's what we might see. We have a packet whose source IP address is this, 192.168.2.100 with the subnet mask of 255.255.255.0. Destination of that packet is this 192.168.100.200 with the subnet mask 255.255.255.0. We convert to binary, we look at the 'ANDing' result. 1 and 1 is 1, 0 and anything else is 0, the 'ANDing' result produces this number right here. That's the source of the packet. The destination of the packet we AND the IP address and the subnet mask and we get this result here. This number does not match this number and actually all we have to look at is the third octet, to reinforce that, these two numbers do not match. So the decision is made to send that packet to the default gateway. If these two numbers matched, then we would send that packet out on the local network and that is how the 'ANDing' process or in other words the logical AND operator evaluates traffic that has been destined for remote networks or the local network.