6. The IP Route Table Routing

hello and welcome in this tutorial we're going to take a look at the IP route table and see how it functions how it enables us to route packets across the network so in order to do that we'll begin by looking at what's inside the IP route table the contents and what key pieces of information are in there and what are they used for and then we'll go ahead and actually look at a routing example and see how the IP route table is applied in order to help us route a packet okay so let's get started so what's inside the IP route table well there's a bunch of information and we're going to begin by looking at the three key components so the first key component is the IP subnet and you'll see a bunch of these in an IP route table and a subnet could represent a group of IP addresses or it could be a single IP address that the router has learned about so it depends it could be a group or it could be a single IP depends how the router learns about that that particular subnet that particular IP address information next to each subnet is an outgoing interface on that particular router and that is used to tell the router if you need to send a packet that matches one of these subnets send it out this particular interface okay likewise also associated with each subnet is a next hop IP address and that next hop tells it well this is exactly where you need to send it to you know which interface to go out but this is telling it what is the next hop in the path in order to get to the final destination what do I need where do I need to send it specifically and that's usually an IP address listed there as well so there's other information like I mentioned there could be information in the IP route table about how the route was learned was it learned via a particular routing protocol was it statically configured perhaps this subnet is directly connected to a router so that type of information is also in there and also sometimes you'll see route metric information so a particular routing protocol uses a metric associated with a route in order to determine the best route and sometimes you'll see that information in there as well for now though we're going to leave some of that more complicated information on the back burner and focus on these key parts of the IP route table so let's go ahead now and actually see how these are used in an actual routing example okay so for our routing example we're going to have PC one send a packet to PC two and these two pcs are in different subnets and they are separated by three routers and so let's trace the path and see what happens at each hop along the way routers one two and three and see what actually happens when it comes to receiving a packet and using the IP route able to get that packet to where it needs to go okay so PC one determines hey this this destination 192 168 2.1 the IP address of pc 2 is not in my subnet so i send a packet to my default gateway in this example router 1 has an interface with the IP 192 168 1.3 that would be the default gateway for pc 1 ok so pc 1 goes ahead sources a packet sends it to the default gateway which is router 1 router 1 receives the packet and as we talked about in other tutorials it's going to go ahead rather one's gonna check the frame check sequence in the ethernet packet Ethernet frame in order to determine if there are any errors on the in the frame during transmission assuming that's okay router one's gonna strip off that layer 2 frames so that you can take a look at the IP address in the layer 3 packet so in this case the packet would have a destination IP address of PC 2 and layer rather router one is going to look at that IP address and then compare it to its IP route table so the IP route table of router 1 will more than likely have many more subnets listed but for our example we just have we just have one and you can see here that router 1 has an entry for this subnet 192 168 2.0 and PC two's IP address falls within that particular subnet next to the subnet router one knows it needs to send that packet out interface serial zero and then finally the next top information is listed and you can see here 10.10 2.1 matches the IP address on the serial interface of router 2 so not only does it know which interface to send it out but it has an IP address specifically the device it needs to send it to in the next hop within the path in order to get to pc - ok so router one will go ahead and say alright well I've got a match and I know where to send it now so I'm going to go ahead and encapsulate this packet into with with a layer two encapsulation and on the serial interface it could be hdl-c or PPP depending on what's in use there and it will go ahead and encapsulate that packet and then send it out on to out serial zero interface and that packet would arrive that frame would arrive at router two and router 2 is pretty much going to do the same thing that router 1 did it's going to go ahead and see if the frame was damaged in any way if there any errors in the frame when it was received assuming it wasn't it will go ahead and remove that layer 2 frame hdl-c or PPP so that it can go ahead and take a look at the IP packet and again the destination IP is listed in the header of the IP packet and that's what router 2 needs to get access to in order to compare it to its route table and figure out what to do with it so here router to route table is going to look pretty similar to router ones except it's next hop information is different so a router 2 goes ahead and says ok I have a packet to send a 192 168 2.1 it looks at the router table sees a match it that IP falls within the subnet listed it knows which interface to send it out and in addition it says okay well I need to send it to the IP address of 10.10 3.1 which is the IP address on the serial interface of router 3 which is connected to router 2 so has all the information needs it finds a match and then router 2 goes ahead and does the same thing it'll go ahead and encapsulate the layer 3 packet in hdl-c or PPP for example and send it on its way so you can see the process is pretty much the same for router 1 and router 2 in terms of receiving the packet getting to the information finding out where it needs to go and then sending it out so now the packet arrives at router 3 and router 3 again does the same thing it checks for errors it will remove the layer 2 frame take a look at the IP packet and here router threes router tables gonna look a little bit different it has the subnet listed and here it has an interface in which it needs to send out packets destined for that subnet in this case though it is the ethernet interface so we've we've arrived at the router that is connected to the subnet itself and so this is a local area network and so it's going to go out the ethernet interface and you'll notice there's no hop no next hop configured in this route table on router 3 and that's because there are no more no more hops left in order to get to the destination when router 3 looks at that IP packet it's going to say ok I see the destination IP and I see it falls into this particular subnet and that's that that subnet is directly connected to me I'm a part of that subnet and so I don't have to send it to anyone else but the actual destination itself so there's no need for a next hop because I finally have direct access now to the final destination for this IP packet so in this case router 3 will go ahead and compare it to the subnet find the interface it needs to send it out on and of course it has the IP address the destination IP in the IP packet and so it knows where to go it will probably go ahead and have find out the layer to MAC address so it'll use the art process in order to find out the the MAC address of PC - it will then go ahead and encapsulate the layer three packet into a layer two Ethernet frame and send it on to the network and finally PC - receives the frame and then it will go ahead and process that frame and so that is the end-to-end example where you can see how the IP rat table is used at each hop in order to figure out is there a match and if so where do I send it and the information on where to send it is listed in the interface and the next top information okay so let's summarize everything we went over we know the IP router table has some basic contents like the IP subnet the outgoing interface on the router and the IP address of the next hop and all three of those are used in order to figure out where a packet should be sent we touch briefly upon some other information that exists in the IP route table and although we didn't get into a deeply here keep in mind there's more information and we cover that that more detailed information in some of the more advanced tutorials and then we took a look at an actual routing example and we saw how the destination IP address is compared to the route table and we look for the best match and then once we find that match the route table tells us where to send it and so in addition to that process of finding the best match and then taking in action what should be really apparent to you is the the importance of understanding IP addresses because this comparison process of taking the destination IP address and finding a match in the IP route table oftentimes you will have to figure out if there's a problem scenario of what's happening on the router and is it functioning properly and and why is a particular route chosen over another route and you if you don't understand the basics of IP addressing and subnetting it's nearly impossible to understand what's really happening so if you haven't yet take a lot of time to go over the I P addressing and subnetting tutorials that is some of the most important information you could possibly understand and need to know on a daily basis okay and then we also took a look at the encapsulation process and what happens you know on a very micro level of each step along the way where a packet an IP packet is encapsulated and it's sent along its way and then when it's received the frame is checked for errors and then that that frame is removed or it's it's D encapsulated in order to get to the IP packet where the IP addressing information is stored and then the router can use that information to go ahead and route all right so that's everything we want to cover in this tutorial on the IP route table and a basic routing example thanks for watching

Channel: System Engineer

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Id: hvuiigb0fKA

Length: 11min 45sec (705 seconds)

Published: Mon Apr 24 2017