 Okay, so to communicate on the network, we need an IP address and a MAC address. Let's focus in on the IP address for right now. When we are sending something on the network, when one device needs to communicate with another, we put what's called a source IP address and a destination IP address in the frame. Now, on a local area network, we must have a MAC address to communicate. We've talked about MAC addresses. So if we're communicating on a local area network, we will know the IP address of a computer. And what we have to determine is we have to determine that MAC address in order to be able to send that frame. That's what the address resolution protocol does for us. Art will basically resolve an IP address into a MAC address. So let's say we need to send a picture from one computer to another computer on our network. This is my picture. What's going to happen is we're going to break this down into smaller chunks of data. We're not going to send that whole picture in one packet. And the reason for that is we have to share the network with other devices on the network. And so when we break this down, we get smaller chunks that we can send a piece at a time. And then we do what's called interleaving in which we share the medium or the wire with other devices on the network. So we'll take the first chunk of our picture. We'll put it on the wire. And then maybe another two, three, four, five, six computers will send a chunk of their data on the wire. And then it will come back around to us and we will send another chunk. Now this all happens in milliseconds. So the user doesn't really recognize that this is happening. But you are sharing that network with other devices. And if we try to transmit at the same time, we have what's called a collision, which there's a whole process for resolving a collision. So what do we need to do or what does the computer do when it sends its first chunk of data? All right, here's the data. But how do we know how to get it to where it's going? How does this little packet tell the network what device this data is meant for? So for that, this little chunk of data will also have an extra piece on the beginning of it called a header. The header is a portion of the frame that contains a source address and a destination address. For right now, we'll just talk about IP addresses. Remember that on a local area network, MAC addresses are used to communicate. But we're going to talk about an IP packet right here. So just for simplicity purposes. So the header will have a source address of 192.168.0.1 and a destination of 192.168.0.2. So basically this little header has information about addressing. Where does the packet, where does this data need to go? What computer is it meant for? What is it destined for? There's also another little piece at the end because how do we know that the data that is sent matches the data that is received on the other end? And for that we have what's called a trailer. And the trailer or the tail of the IP packet has a couple of things in it. One is called a CRC or a cyclic redundancy check. What this does is basically tell us that there are no errors. So basically CRC equals error checking. Then there's also at the very, very end there is an end of frame delimiter. And the end of frame delimiter basically tells us that we're at the end of the frame. That there is no more data to transmit. You have to remember that we are using wires to transmit this data. So a packet when it is put onto the wire is just a stream of electricity. And there's dips and spikes in that electricity. And a dip in the electricity represents a zero whereas a spike in the electricity represents a one. So we need somewhere in this frame where we have an end of frame delimiter. And an end of frame delimiter is a specific sequence of zeros and ones. A specific sequence of spikes in electricity and dips in electricity that tells the device that this is the end of this frame transmission. So that's called the end of frame delimiter. Now I want to talk a little bit more about this error checking and how that actually works. When we have this data prepared at the source we run what's called an algorithm on this data when we are encapsulating it or packaging it up into this frame. That algorithm will give us an answer or a result. Actually it's a mathematical formula it gives us an answer. So let's say that answer is one two three three four five six. That's terrible three. One two three three four five six. Let's say that's the answer to this. Well when this packet gets to the destination and it's being decapsulated on the other end the receiving machine will run the same algorithm on that data. And we should expect to get the same answer. If we run the algorithm and we get the same answer that's in the end of the CRC then we know that this data arrived in the state in its original state that it is intact. That it we received it as it was meant to be received it hasn't been tampered with and it has not had any kind of data loss or error in the frame. If we get a different answer then we know that there is a problem in the frame. Either the frame has been tampered with or there's been some sort of collision there's some sort of problem with the communications on the network that has caused this frame to become corrupt. At that point in time the receiving computer would discard the frame and request that frame again.