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The life of a Packet

5. ARP for Routed Traffic

In the last lecture, you learned about ARP, the Address Resolution Protocol, and how it's required to allow communication between two hosts over an Ethernet network. In the last lecture, you saw how it works when both hosts are on the same IP subnet. In this lecture, we'll cover how it works when we're on different IP subnets. So you already know that that means that the traffic is going to have to go through a router. So we'll work through an example for this. In this example, 172 23 4 1 wishes to send a packet to 1 9 2 1 68 1 0.

So that's two different IP subnets. You see the sender over there on the left, and the receiver is on the right, and we've got a router that is going to route traffic between those two subnets, so we can have artwork the way it did earlier, when both hosts were on the same subnet. If the sender on the left sends out a normal art request for one thing, say, two things, one thing, or 6810, that will go out as a layer to broadcast, and it wouldn't be forwarded by the router, so the art request would never get to the receiver. So obviously that isn't going to work.

Also, we know that when we send traffic between two different IP subnets, it has to be sent via a router. As a result, the sender will not send an art request for one nineteen sixteen. It knows not to do that because it compares its own IP address and subnet mask with the destination IP address, and it can see that it's on a different IP subnet. So the sender knows that it has to send the traffic via a router. As a result, it does not send an ARTREQUEST to the final destination. It sends an art request to its default gateway. So the sender at 172 23 4 will send an art request to 107 223 4254, which is its default gateway that originates from the sender's source Mac. As is customary, the destination Mac is for an art request to broadcast address.

And in the art request, it says it's a request for 107 22342 five, asking it for its Mac address that will hit everything in the 17223-4-dot-O subnet, including the router. The router will see that it's an Arc request for itself and will send an ARP reply that comes from its source Mac address of 107-223-4254, which was four five-dot-six, and the destination Mac address is one two three; the router knows to send it there. Because that source Mac was in the original art request, the sender will have been holding the IP packet that's intended for the final destination while it sent out the original art request there. It now knows where to send it for the destination Mac address, so it will now send that IP package.

The IP packet will originate from the sender's source IP address, which is 172, 234, 112. The destination IP address is the IP address of the final destination. So that will be 192-16-8101. The source Mac is 1, 2, 3, and the destination Mac is for that sender's default gateway, which was 4, 5, 6. So that packet will hit the router, and the router will see it, but it needs to send it to one thingtwo, dot one, six, eight, dot ten, dot one. In our example, the router is unaware of the mac address 192-16-8101 because it has never communicated with it. So it's not in the art cache. As a result, the router will hold the IP packet from the sender on the left and send an art request for 192-16-8101 to its interface on the right, which has an IP address of 19216-810-2-4. So it's in the same IP subnet as the final destination.

So we'll call it a one-in, two-in, and 6810-in art request. It is derived from the router's IP address's source Mac, 19216 810, two hundred and forty-four. So that was Mac address four, dot five, and dot seven, and it's an art request. So it goes to a destination Mac of layer 2 broadcast F that will hit everything in the one thing two one 6810 subnet, including the receiver over on the right. The receiver on the right will recognise it as an artrequest for the IP address 19216-810-1. As a result, the art response will be sent. The art response originates from a source mark of 2, 3, 4, and it travels to the destination Mac, the router's interface on the righthand side, which was 4, 5, and 7. The router now knows the Mac address of the final destination on the right, so it will send the IP packet. The IP information in the packet never changes. The source IP address is always the original sender, which is 172.23.4.1 in our example.

And the destination IP address is always the final destination address, which in this case was 19216-810. The source Mac is the router's interface on the right hand side, which was four, five, and seven, and the destination Mac is two, three, and four. so you may be noticed. In the example, let's go back a slider. Like I said, the source and the destination IP addresses never change from end to end. But the Mac address, source, and destination will change physical hop by physical hop.

We'll have another look at that in more detail in the next lecture as well. Okay, so that is how the art of routing traffic works. If you want to view the art cache on a Cisco router, the command is simply show ARP, and to clear the art cache, it's clear ARP cache. Let's have a look at that now in the lab. This is a continuation of the last lab, where we covered DNS and where we had R1 at 1010 one.R 2 is at 1010, and R 3 is at 1010, 21. So, if I jump onto R One here and do ashow ARP, it'll have already done two pings, R One and R Two.

And let me just make this full-screen and do it again to make it show up a bit more cleanly. And you can see that it's got entries here for 1010-1, where you can see the Mac address and that it's reachable out of the interface via fast ethernet, and also for 1010-2, which has the Mac address and the interface that it's reachable out of. We could also have a look at R-3 as well.

So let me just bring that back up again. And if I go on to R-3 and do a show ARP on here, I need to go to enable prompt first, let's say end, and then show ARP. And R3 is in the subnet 1010-20. So on there, let me just make this full screen again so you can see it a bit more clearly. You can see it's got entries for 1010-21 and 1010-22. So that's how you see the art table on a Cisco router. It's as simple as showing up, which will show you the IP address, Mac address, and interface that is reaching.

6. Life of a Packet Example Part 1 – DNS

This lecture is a big deal. It's the culmination of everything that we covered already, and we're going to walk through the life of a packet from the sender through to the receiver and how everything works together to make that possible. So really, it's a review of what we've already covered already. But this is going to tie everything together, and it's going to prove to you that you now know the fundamentals of IP networking.

So in the example here, we've got it hosted on the left, and it's going to send some HTTP traffic to our web server over on the right, which is www.flatbox.com. It's going to use the FQDN to send back traffic. So it's also going to need to resolve that FQDN name to the IP address using DNS. Also, it's a routed network. We've got different IP subnets there, and we've got a couple of routers. In the middle of the topology, I've deliberately included multiple subnet routers and a DNS server because I want it to mimic what you would see on a real-world network because I want you to be confident that you can work on real-world networks and that you understand how IP networking actually works because of that. There's quite a bit to this example. So it's going to take a little while.

So I'm going to split this into two videos. The first part will cover resolving the FQDN to the IP address by DNS, and the second video will cover the HTTP traffic. Okay? So let's walk through how this is going to work right from the start. So again, we're going to use the OSI stack model for this. Don't worry, this is pretty much the last time you'll see it in the course now, but it's really fundamental to how networking works, so that's why you've seen it so many times. So one more time, in the OSI reference model, we're going to be composing that web traffic and sending it to the web server from our source on the left.

So it will compose the packet starting off with the information from the application layer; that will then be encapsulated in the presentation layer header, and then that will be encapsulated in the session layer header. Then we get down to the really important information for networking. So layering, for the transport layer, this is web traffic, so it's going to be sent with TCP, and the destination part is going to be part 80. Then, when the sender on the left is composing this web traffic, it needs to make the layer4 header next, where it needs to enter the destination IP address, and it doesn't know what the destination IP address is because the user just opened up a browser and entered in there, "www.flatbox.com." So the sender will need to resolve that to an IP address to complete this package, and it's going to use DNS for that.

So Hostage, which is at 1010, 24, wants to send a packet to the FQDN of www.flatbox.com, but it doesn't know the destination IP address, so it will hold on to that packet. And in the meantime, it will send a DNS request to its DNS server at 1010 110. So the host already knows its IP address, its subnet mask, its default gateway, and its DNS server. Hosta will compare its IP address and subnet mask to the destination address of the DNS server, and it will see that it's on a different IP subnet.

So the DNS request will need to be sent via its default gateway. Host A will hold the DNS request and send a broadcast A request for its default gateway, which is at 1010 One. So you can see that in the diagram here. host sends an art request. It comes from 1010. It says it's looking for its default gateway at 10. says, "Hey, what's your Mac address?" So that comes from a source Mac of 1 2 3 and goes to the layer 2 broadcast address of F F. The art request will be received by Switch One on the left. Switch One will add an entry to its Mac Address table mapping the hostage Mac address of One, Two, and Three to Part One. Switch one will then flood that broadcast traffic out to all parts except the one on which it was received. So that will go out in part two.

The art request is still from 1010. We're looking for a 1010 one with a source Mac of one, two, or three. a destination Mac of F that will connect to the router as 1010 One interface Router A will process the art request and see that it is for itself. It will then send a unicast ARP reply back to HostA, and Router A will add an entry for HostA's mapping IP address of 1010 to the Mac address of 1, 2, and 3, and that will be added to its ARP cache. It will then send the ARP reply. Switch One will receive that, and it will add an entry to its Mac address table mapping Router A's Mac address of 4 5 6 to port two. Because the Art reply is a unicast reply and the switch already has Hosta's Mac address in its Mac address table, Switch One will send the Art reply out only port one, which Host A is plugged into.

It is aware that it is out there. Okay, so there goes the art reply. It says I'm 1010 One, and here's my Mac address that came from Router A. The time on the source Mac is 4:56, and the time on the destination Mac is 12:30 n. Host A. Hosts will receive that. It will then add a Router A entry mapping Router A's IP address of 10 to Router A's Mac address of 4 5 6. It will add that to its ARP cache, and it will then use that whenever it needs to send traffic to another IP subnet. Host A will then send a DNS request for www.flatbox.com. So that DNS request will say, "Tell me the IP address of www.flatbox.com, please." It comes from 1230 a.m. host A's source Mac. It is directed to the hostagedefaultgateway.macaddresses four, five, and six.

The source IP is 1010 on Hostea, and the destination IP is its DNS server at 1010 100dot 10, which is unique as traffic. So switch one will send the DNS request only to port two, which Router A is plugged into and which the switch already has in its Mac address table. So the DNS request will come to Router A. It will receive the request and see that the destination IP address is 10 100 dot 10. The DNS server, Router A, has an interface in that subnet: 1010 100 00:24. So it knows that the destination should be available at that point. It doesn't know the Mac address of 1010 110 yet, though, so it will hold the DNS request packet and send an art request out of the 1010 101 interface. So there goes the art request that's from 1010 101 on the router.

It's searching for 1010, 110, and asking for Mac's address. It comes from an eight nine eight Mac source on the router's interface. The destination map is always the same for an art request for the layer 2 broadcast address. The art request will be received on switch three. Switch three will add an entry to its mapping router's Mac address table as Mac address eight nine, A two parts one. It will then flood the broadcast traffic at all parts except the one where it was received, so that it will also reach the DNS server out port two. Again, the art request looks the same. It's 1010 101 looking for 1010 110 from a source Mac at 8:09 A to a destination Mac of F F. So the A request hits the DNS server's interface at 1010, 110. The DNS server will process the art request and see that it is for itself. It will then send a unicastArt reply back to Router A.

The DNS server will add an entry for Router A's IP address of 1010 101 to Mac address 819 A to its cache, and it will use that whenever it needs to send traffic to another IP subnet because 1010 101 is its default gateway. So there goes the Art reply from 1010 to 110, saying, "Here's my Mac address of source Mac 3; 4; 5 going back to the router at destination Mac 8; 9." Switch three will receive that, and it will add an entry to its Mac address table, mapping the DNS server's Mac address of three, four, and five to part two. It will then send out only part of the art reply, which Router A is plugged into because that is a unique house reply and it already has Router A in its Mac address table. So there goes the art reply, unchanged, on its way to Router A. Router A will receive that. It will then add an entry in its ArpCash for the DNS server mapping IP address 1010:110 to Macaddress 3:4:5. Router A will then send the DNS request it was holding from Host A to the DNS server.

The source and destination Mac addresses of a packet are updated hop by hop, but the source and destination IP addresses always remain the same end to end, unchanged from the original source to the final destination. The source and destination Mac addresses in our example will be updated to come from RouterA and go to the DNS server. for this DNS request. The source and destination IP addresses are still hosted at 1010, and the DNS server is the destination at 1010 100. So there goes the DNS request. A DNS request is saying, "Tell me the IP address of www.flatbox.com." The source and destination Mac addresses are now changed to 8-9-A on Router A going to the DNS server. The source IP is still 1010 on host A. The destination IP is still 1010:110 on the DNS server. Switch Free will send out only Part 2, which is the DNS server plugged into it, which Switch Three already has in its Mac address table.

So the DNS request gets sent down to the DNS server. The DNS server will receive the DNS request packet and see that the destination is itself. So looking at the OSI stack again, it comes in on the physical wire, and the receiver will then process the package starting at the bottom of the stack and working its way up. So it is clear that the destination Mac addressed was three, four, and five, which is itself. So it will carry on processing the packet. It sees that the destination IP address in the layer-3 header is 1010 110, which is itself. It will carry on for purposes in the packet. Then in the layer for the transport header, it sees that it's UDP and that it's on port 53. So it knows that this is a DNS request. because DNS uses UDP port 53.

The packet will then be passed up the stack. So look at the session header, the presentation header, and the application header, and it will process that DNS request. The server will look in its DNS database and see an address record for www.flatbox.com at 1010:1210 that was configured in DNS. It will send that information back to Host A in a DNS response. It knows to send the response to 1010 because that was the source IP address in the DNS request, and it knows to send it via Router A because Router A is its default gateway and the destination is in another subnet. The DNS server already has the router's Mac address in its art cache, so it does not need to send an art request for this. So the DNS reply says that www.flatbox.com is at 1010 1210.

Three, four, and five are the source Macs. At eight nine, the destination Mac is the default gateway. A source IP is the DNS server at 1010110, and the destination IP is hosted at 1010. Switch three will receive a DNS response, and it will send out only part one, which router A is plugged into and which it already has in its Mac address table. So it passes that on to router A. Router A will receive the DNS response packet and see that the destination IP address is 1010. It has an interface in the subnet of 1010 or 24, so it knows that the destination should be available at that port. And router A already has the Mac address for 1010 in its cache. Because it does not need to send another art request, it will send the DNS response through that interface.

Again, it is going from source IP 1010 110 to destination IP 1010 Host A via DNS server. So that doesn't change, but the sources and destinations on Mac will be updated. The source Mac addresses are four, five, and six, and the destination Mac addresses are one, two, and three, which correspond to the Mac addresses on the left side of router A. Switch one will receive the DNS response and send out only part one, which indicates which host A is plugged into and which it already has in its Mac address table.

As a result, it sends the DNS response to host. Host A now learns from that DNS response that www.flatbox.com is available at 1012 dot ten. It can now update the packet it was waiting to send to www.flatbox.com. With that destination IP address, host A sees that the webserver is not on its own subnet, so it knows that any package sent there must go via its default gateway. Okay, so at this point, host A has learned the IP address of the Web server through DNS. So that covers us for part one of this lecture. Part two is in the next lecture, where you'll see how the actual HTTP traffic makes it over.

7. Life of a Packet Example Part 2 – HTTP

Looking at the web packet that the host was holding before, it had been able to compose it as far as layer four, but it couldn't put the layer three IP header on there because it didn't know the destination IP address yet. It just received that from the DNS server. So it can carry on composing that packet. It knows that the destination is 1010-1210, and it sees that that is on a different IP subnet. So it knows that the destination Mac address is its default gateway, which already knows that it is four, five, six. It will then put that web traffic onto the physical wire. So here's our HTTP get request. The source Mac is one, two, three. The destination Mac is the default gateway four, five, and six. The source IP is its own IP address, 1010, and the destination IP is a web server at 1010-1210 that will hitswitch one, which will send a packet to Router A, which it already has in its Mac address table.

The packet will come into Router A. It shows that the destination IP address is 1010 1210. And in our example, router A does not have any interfaces in the 1010-1224 subnet. So in that case, it's going to take a different route to get there. The route can be either statically configured by an administrator or learned dynamically through a routing protocol, which is also configured by the administrator. Later lectures will go over how to configure static routes and routing protocols with COVID. So for our example, let's say that the administrator has already configured a static route for 10, 10, 12, and 24 with a next hop address of ten, 1011, and 2, which is on the next hop router. Router A has an Ethernet interface in the 10110 subnet. It doesn't know the Mac address for the next hop address of ten 1011 2 yet, though, so it will hold the HTTP packet from Host A, and it will send an ARP request out that interface and to the ten1011 subnet looking for ten 1011 2. So there goes the art request.

It comes from 10:11:1, and it is looking for 10:11:2. What's your Mac address? It originates from a source Mac of five, six, and seven, and it travels to the layer to the broadcast address of f dot f dot. The art request will hit routerbaseinterface at 1011, and it will see that the art request is for itself. It will send a unicast AR reply back to router one. While this is happening, Router B will create an entry for Router A, mapping the IP address 1011 to the Mac addresses 5/6/7. So the art supply is returned. It says, "Hey, I'm ten 1011 two, and here's my Mac address of six seven eight going to the destination Mac of five six dot seven." Router A will get that information, and it can now forward the HTTP packet it was holding to Router B. So it's the original HTTP get request from host A. The source IP is always the same, so it's still 1010 on hosta going to the destination IP of 1010/1210 on the web server.

But the source and destination Mac addresses will be updated for this hop. As a result, the source for Mac is 5:6:7. Mac's destinations are six, seven, and eight. Router B will receive the HTTP packet and see that the destination IP address is 1010 dot twelve dot ten. Router B has an interface in the subnet of 1010-1224, so it knows the destination should be available at that point. But it doesn't know the Mac address of 1010-1210 yet. So it will hold the HTTP packet and send an ARRAY request out. That equates to ten 1012 interfaces. So there goes the art request. It's from ten 1012 one looking for ten 10 12 and asking for the Mac address that comes from a source Mac of seven eight, dot nine, going to the FDA F F dot layer two broadcast. The art request will be received by switching to Switch will map router B's Mac address to an entry in its Mac address table. It will then flood the Art request broadcast traffic at all parts except the one it was received on, so that we can send out of part two.

The art request will hit the webserver as an interface at 1010 and 1210. The web server will process the art request and see that it is for itself. The web server will then send a unicorn art reply back to Router B, and it will add an entry for Router B mapping an IP address of 10, dot 12, dot 1, to the Mac address of seven, eight, dot 9, to its art cache. That's the default gateway, so it will use that whenever it needs to send traffic to another IP subnet. As a result, the Art response will say, "I'm 1010, 1210, and here's my Mac address of 2 3 4 going to the destination Mac of 7 8 9." Switch two will get that and add an entry to its Mac address table mapping the web server's Mac address of two, three, and four to part two, and switch two will then send the Art reply out only for part one, which Router B is plugged into and already has in its Mac address table.

So there goes the art reply, unchanged from the web server. Router B will get that and add an entry for the web server mapping IP address 1010-1210 to Mac address 2-3-4-5 in its art cache, and then Router B will send the HTTP request it was holding to the web server. So again, it's the original source IP on host A 1010 and the original destination IP on the web server of 1010 1210. The Mac addresses will get updated with the source Mac of seven, eight, and nine and the destination Mac of two, three, and four switches. The switches will send out only part two of the HTTP request, which the web server is plugged into and which the switch already has in its Mac address table. And finally, the HTTP Get request will reach the web server, so it comes in on the physical wire. The web server will look at the layer2 header and see that the destination Macaddress is two, three, and four, which is itself.

As a result, it will continue to process it. It will look at the layer 3 IP header and see that the destination IP address is 10 10 12 10, which again is itself. It will carry on processing it. It will look at the only Layer 4 transport header and see that it is TCP 80, so it knows it's web traffic. It will then carry on up through the session, the presentation, and the application layer, and the web traffic has now reached the web server.

Okay, the art and macro tables are already built, so any subsequent packets in either direction will flow without any need for our requests or switching flooding. So let's say the second packet in the session goes from hosted to unhosted, so it will send the HTTP Get request. It travels from a source Mac of one, two, and three to its default gateway's destination Mac of four, five, and six (source IP 1010, destination IP 1010, 1210). It already has the destination Mac address in its cache, so it can just immediately send the packet that will get to router A, which also already has everything in its cache.

So the HTTP Get request is still coming from source IP 1010 going to the web server at 1010 1210; the Mac addresses will be updated to be relevant for this hop, which had a source mark of five, six, and seven and a destination mac of six, seven, and eight that will hit router B. And, once again, it has an interface in the subnet, usually dot twelve; it also knows which interface to send it out of the source IP, which remains unchanged.

Ten destination IPs (10, 12, 10), the source Mac is updated to seven, eight, and nine, the destination Mac is updated to two, three, and four, and we have end-to-end traffic. OK, so we got there; we covered the complete life of a packet from end to end, and you now have a really good knowledge of how IP networking works. Honestly, there's really not much more to it than that. The following lectures will go over the various features and functions available on our routers and switches to support that, as well as how to configure them.

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