Transcript for:
The Network Layer Explained

hi welcome to class my name is don lafond professor don and this week in cisco 1 we are covering module 8 the network layer if you have any questions please hold those questions for the end of the presentation if you're here with me live ask your questions in the netacad help discussion forums if you if you're in my netacad class currently and if you're watching this on youtube ask your questions down below in the comment section and i'll do my best to swing by every now and then answer those questions although i encourage you to answer each other's question all right great let me go ahead and share my screen a couple things i have to do here to set this up one moment start the presentation and one more step all right great module 8 the network layer all right so what are we going to learn in this module well i am going to explain how the network layer uses ip protocols for reliable communications i'll explain the role of the major heading fields in ipv4 and ipv6 i'll explain how network devices use routing tables to direct packets to the destination network and i'll explain the functions of fields in the routing table of a router network layer characteristics remember the network layer is layer 3. of the osi model and at this layer we the network layer provides services to allow end devices to exchange data ipv4 and ipv6 are the principal network layers communicating at this layer the network layer performs four basic operations addressing and end devices encapsulation routing d and d encapsulation so at this layer we are adding our ip source and destination uh information to the data stream that's coming down the osi model from the application layer and you can see encapsulation happens at four and goes up through the final step we're going to let that run one more time so data is a stream through layers seven six and five when it gets to layer four we get encapsulation uh it turns into a segment there we add in the ip address and it becomes a packet at layer two we encapsulate the mac information into it we learned that last week and last module and that mac information is for next hop and next hop uh each device has its own mac layer and it changes from one hop to the other to the next one but remember the ip address what we add at layer 3 that stays the same from the beginning to from the source to the destination ip encapsulation ip encapsulates the transport layer the transport layer segment this is layer 4 and it encapsulates that layer segment uh adding either ipv4 ipv6 information uh the um neither either works at uh layer three and it doesn't impact layer four that in the data stream and the and the um segment that's coming from layer four it doesn't care if it's ipv4 ipv6 it doesn't matter we encapsulate it with the ipv the ip the ip source and destination at layer 3 the network layer ip packets will be examined by all layer 3 devices as it transverses the network the ip addressing does not change from source to destination and the note here says that's not exactly true because inside of networks uh such as your college network inside of the network we have nat a network address translation it allows for ipv4 private addressing which truly expanded the amount of ipv4 addresses that were available from 4.3 mil a billion to many many more obviously that's how we continue to use ipv4 addressing but um uh there's a typo there there's no such i have to fix that okay hang on a second i can't leave that there is no such thing as ipv5 at least in networking we don't use ipv5 so that was a typo it in ipv4 not does change the address for the they call it for the final mile from the edge router your gateway uh to the post pc that's sending the information that's private addressing and we'll learn that uh in uh in a couple weeks in a couple modules all right ip is meant to have low overhead and may be described as connectionless best effort and media independent ip is connectionless ip does not establish a connection with the destination before sending a packet second there is no control information needed synchronization acknowledgements there's no handshaking and we'll learn about handshaking as we move forward in this course the destination will receive the packet when it arrives but no pre-notifications are sent by ip if there is a need for connection oriented traffic then another protocol will handle it typically for example it's tcp at the transport layer which we'll learn after we master ipv4 and ipv6 at the layer 3. i want to add that this is similar to the post office when you when you address a letter and a standard letter 50 stamp and you put it in the mailbox you have no control of that letter once it leaves you don't know if it gets to the destination the destination does not reply saying that it has received the mail they might they if it's grandma she might call and say thank you so much or if you get uh well i don't know if that happens anymore but if grandma sends you a gift in the mail you want to call her and say thank you right so uh it's just like regular mail not like ups or fedex where there's tracking or even uh priority mail with the ups with this tracking you can see it that would be added at layer 4 if it was a network at this layer layer 3 it just sends the information it's responsible for sending the information adding the source and destination packet information source and destination addressing information ip is best effort i p does not guarantee delivery of the packet ip has reduced overhead since there is no mechanism to send data that is not received ip does not expect acknowledgements ip does not know if the other device is operational or if it received the packet and in this diagram here they they show three packets of information being sent by the host and it trans transversely transverses the network in many different ways and it gets to the other side and if there's only two packets there's only two packets there's no mechanism to reorder it or even to resend those packets that happens at a higher level ip is unreliable i p cannot manage or fix undeliverable or corrupt packets undelivered or uncorrupt packets ip cannot retransmit after an error ip cannot re realign out of sequence packets ip must rely on other protocols for these functions ip is also media independent ip does not concern itself with the type of frame required at the data link layer or the media type at the physical layer whether it's fiber copper wireless ip can be sent over any media type and you can see it here it leaves the source host as copper gets sent to copy copper serial over here it's fiber and at the last mile it is a wireless no problem a network layer will establish the maximum transmission unit mtu network layer receives this from from control information sent from the data link layer the network then establishes the mtu size fragmentation is where when layer three splits the ipv ip4 packet into smaller units fragmented fragmenting causes latency ipv6 does not fragment packets example a router goes from ethernet to to a slow wan with a smaller mtu i like to explain it this way let's say you call tech support and you guys are all techy people you call tech support and you're having a problem with your wireless it's not connecting wired devices and you have already checked some things and you think your route is bad so you give a call uh to uh the tech support in your and you just give it all to them yeah you just give it all to them right you you tell what the problem is you tell them what you troubleshot you tell them uh where you think the problem is and the response you get on the other end is um sir may i have your name uh my name's don lafond and what router do you have and you give them uh xyz one two three uh slow down x y z one right and then you go on and they don't wanna know everything uh they want you to slow down right well that would be um uh that would be fragments right it would be you have to go and i'm sorry that's an example of mtu's as a smaller mtu size you have to transmit less information at one time other technicians usually maybe at a layer two tech you get a hold of them and you use blah you give it all to them and they're like oh yeah i know what that problem is let's uh i want you to sign into this and sign into that and then you have to slow them down if they're asking you to do things that you don't understand you can also control the mtu size all right hopefully you understand mtu now uh if you didn't before um ipv4 packet uh the ipv4 ipv4 is the primary communications protocol over the network later it's not the newest ipv6 is but it has been around for a long time uh and it it's not the only one that we've had there were there were um ip protocols prior to uh ipv4 uh like nor novell um i i didn't learn overall so i'm not familiar with it uh but that was a a type of um ip protocol that existed before i ipv4 so it's not the newest it's not the oldest the network header has many purposes it ensures the packet is sent in the correct direction to the destination it contains information for the network layer processing in various fields and we'll go over those fields in a moment and the information in the header is used by all layer 3 devices that handle the packet this is the ipv4 header and i can and i'll go through each field and not the ones in gray those aren't critical at this point uh but the version it's uh well let me let me explain some characteristics so first of all uh it is in binary it's ones and zeros you know that uh it contains several fields information uh the the diagram is read from left left to right top to bottom and the two most important fields obviously are the source and destination ip addresses now let's go through each field so uh the version uh this is uh the uh this tells uh the this such it starts off the packet saying it's an ipv4 an ipv6 um packet if it's ipv4 it's zero one zero zero if it's ipv6 it's zero one one zero that just tells the compute the receiving computer what type of uh information it is the ds is used for quality of service and uh you'll learn more about that in cisco three uh header checksum directs corrupt detects corruption in the ipv4 header uh time to live that is the number of a hops that the packet will make before it is automatically dropped each hop between router to router i think mac address is changing right as it moves from router to router each hop decrements it by one when it gets to zero it is just dropped protocol that is the next level ups protocol so if it is an icmp packet tcp udp et cetera that is identified in the pro protocol field and the source and destination ip address is also required at this level uh in the i'm sorry in the header all right and this is what i was just reading now you uh are going to see a video now on ipv4 header information let me open that right now by the way if you're watching this class online uh you can get these same videos by going to itexam answers.net this software has all of the content and all of the videos that are found inside a packet tracer so i encourage you to go check out these videos if my recording of a recording isn't very good just go download and watch uh the video live all right let me maximize this i have a screenshot traffic and you can see that the second packet that's been captured has been highlighted and then in the packet details window the network layout i'm going to increase the volume a little bit hang on a second information has been expanded to show us all of the things happening at the network layer so let's see what's happening in this particular package that we're examining we can see that first of all the network layer protocol or internet layer protocol that we are dealing with was internet protocol version 4 ipv4 we can also see that the source ip address was 192.168.1.109. you can see it also highlighted up here in the packet list window area and that the destination ip address was 192.168.1.1 and we can also see that up here we can see that at the higher layer this is a tcp protocol packet but if we limit ourselves to just the ipv4 fields or the ipv4 information we can see the different types of control information that's contained in every ipv4 packet for instance the version number which is four identifying this as an ipv4 as opposed to ipv6 packet the header length or the length of the header this is the minimum size of an ipv4 header the differentiated services field which is used for packet prioritization and is useful for applications like voice over ip the total length of the packet the identification number which is used for fragmentation the flags you can see that the df bit has been set which stands for don't fragment this packet is not large enough or is not identified for fragmentation uh fragment offset the ttl or time to live which is set to 128. every time a packet is routed from one hop to the next the ttl number is reduced when the ttl number reaches zero the packet is dropped ensuring that packets don't circulate on the internet forever on an endless loop the ttl value is also used in icmp trace routes and pings the protocol field lets us know the type of information to expect in the data portion of the packet a 6 identifies the data portion of this packet as being a tcp packet the header checksum field which allows routers to check to see if there are any errors or inconsistency in the ip header if there is the packet will be dropped and then lastly the source and destination ip addresses which are the most important part of the ipv4 packet let's take a look at two more screenshots of wireshark packet captures and we'll see some similarities and differences the next screenshot shows us that now we're looking at the eight packet capture the packet's source ip address is also 192.168.1.109 and the destination ip address is 192.168.1.1 except this packet is an http get request so this is a request to a web server located at 192.16 you can see that the network layer or internet layer information has been expanded that it's also the ip version 4 protocol and that we have similar information in the different fields notice under the total length field that this packet is 411 bytes compared to the previous packet which was only 52 bytes you can tell that this packet has a lot more information or is much larger packet than the previous one if we look below the internet protocol version 4 information we can see the tcp information and then below that that there's hypertext transfer protocol or http protocol information in this packet as well i'll move forward to the next packet and you can see that this packet is the 16th packet captured right up here it's also from host 192.168.1.109 to host 192.168.1.1 except this is the icmp protocol you can see from the information in the packet list window that this is an echo or ping request if we look in the internet protocol version 4 information in the details area we can see some minor differences the version is still four the header length is still 20 bytes but we can see that the flags are slightly different and that the protocol field is now set to one indicating that the data portion of this packet is an icmp protocol message notice that in the details window at the bottom here is an expanded area to look at the header information specific to icmp okay so not too scary um it took me a couple i don't know years to remember all of those fails so don't panic uh if it seems a lot uh to remember you will have plenty of opportunity to work on those uh learning the different fields and uh at least one packet tracer which you will be viewing them in this class so let's talk about ipv6 uh now the reason we have ipv6 is there's several limitations of ipv4 the major one is that we're running out of addresses so uh there was only 4.3 billion ipv4 addresses so we have we have run out of those addresses uh there's also a lack and we've replaced it with nat which i kind of mentioned earlier uh there's also a lack of end-to-end connectivity to make ipv4 survive this long private addressing and nat network address translation were created this ended direct communications with public addressing so rarely do you find a host computer with the public address unless it's something like a server for example that you want to be able to be directly accessible from the outside world also it increases network complexity nat was meant to be a temporary solution and uh and creates issues on the network as a side effect of manipulating the network headers head network header addressing and that causes latency and troubleshooting think about every translation from a private address to a public address needs a cpu uh to be involved in a router so obviously there's just going to be that moment difference you out of a bunch of moments and now you've got a real latency here they show you ipv6 uh and the graphic on the right i like a lot because it shows you what uh ipv4 has has uh 4.3 billion addresses as many has and this is what um ipv6 has it has 340 undecillion addresses and that's how many zeros you have after the the ten thirty six i think that says and uh the uh they like to i like to say uh that there's enough uh addresses in ipv6 for an individual ip address for every grain of sand on the planet i've also been uh we did some playing around and we found out that you can there you would have several trillion addresses for every human on the planet uh so for example each one of your blood cells could have its own ip addressing and maybe it would report to a human a computer in your body that would discard damaged blood cells who knows right i used to say we'd never go through all of those uh addresses but if you get ip ip for every uh blood cell then it is definitely something uh that uh we could start working on i'm sure the people that created ipv4 said you know when they had you know 20 computers they were like 4.3 billion addresses like when are we ever going to use all of those addresses but of course we know better here in my home alone i have 46 uh ip ipv6 addressed elements like uh the internet of things elements like my uh my hue light bulbs and my my um my uh range and my dryer and heck my car has its own ip address it is uh tesla and so it um i can talk to it anytime i want using that ipv6 address uh ipv5 ipv6 is a global is globally addressable so it eliminated eliminates the need for nat in ipv4 because everything you can look at the side of my light bulb and you can read the ipv6 address that's just stamped on it so when that light bulb gets thrown away that address will be forever gone from the world but who cares we got 340 billion addresses this is the ipv6 header it is simplified but not smaller in size the header is fixed at 40 bytes or octets long it's several ipv several ipv4 fields were removed to improve performance the ipv4 fields that were removed include flag flag fragment offset and header checksum remember we don't fragment ipv6 so the different steps remember the version i already told you version is 0 1 1 0 which is 6 in binary the traffic class is used for qos information flow label informs the device to handle identical flow labels the same way so when the first flow label comes through it is processed when the next one comes through if it's identical it's just processed the same way so there's no you save some time and latency in that case uh the paid load length is obvious it's a 16-foot 16-bit field indicating the length of the data portion or the payload of the ipv6 packet the next header is is um it tells uh the the uh the router uh what the next level protocol is ipv icmp tcp udp much like ipv6 hot limit is a hop limit does the same thing oh what it does the same thing as ttl but it just does it in a little bit different way basically allowing stopping ipv6 packets from forever transmitting forever transversing the internet and then you have your ipv6 source and destination addressing uh here uh ipv6 packets also contain extension headers extension headers uh characters provide optional network layer information they're optional they're placed between the ipv6 header and the payload and it may be used for fragmentation security mobile support et cetera and it's beyond the scope of this class we don't talk about it and we don't teach it in in cisco one so don't worry too much about that just know they exist unlike ipv4 routers do not fragment ipv6 packets and we have a video video 8.3.5 let's go pull that up three three five this screenshot shows a packet capture using wireshark and the network layer information from an ipv6 conversation let's take a look at in this screenshot we can see that the highlighted packet is packet number 46 and that the source address up here in the packet list window shows that it is a global unicast ipv6 address you can see this starting with the 2001 colon 6f8 the destination address is also a global unicast address 2001 6f8 900 and so on and if we look over in the protocol field we see that at the upper layers this is a tcp packet and then it's an attempt to establish an initial communication with an http web server if we look down in the network layer information area you can see that the ipv6 information has been expanded let's take a look at some of the protocol field information for internet protocol version 6. first of all you can see that the amount of information in the ipv6 header is much smaller than in the ipv4 header now there are some interesting features for one you can see that the version field is the same in this case it says six identifying this packet as ipv6 we can also see the binary six here the next field is the traffic class field the traffic class field serves the same function as the differentiated services field in an ipv4 packet it handles traffic prioritization and congestion the next section you can see is the flow label the flow label field is a new field for the ipv6 protocol its purpose is to maintain the same packet flows through routers and switches so as to help real-time applications that need packets to arrive in the same order you can see the next field is the payload length field this is the same as the total length field in the ipv4 header this field tells us the total size of the packet in this case 40 bytes the next header field serves the same purpose as the protocol field for ipv4 you can see that it's identifying that the upper layer data portion of this packet is a 6 or tcp the hop limit serves the same function as the ttl field in an ipv4 packet you can see that the hop limit currently is set to 64 hops once this decrements to zero the packet will be dropped next we have the source ipv6 address the destination ipv6 address and then at the upper layer we can see that this is a tcp packet with tcp header information let's take a look at the next screenshot in the next screenshot you can see that we've now highlighted packet number 49 and now we have a connection with this web server this packet is now a get request to the web server if we look down in the expanded internet protocol version 6 packet details window we can see that the payload length is a lot larger we can see below the ipv6 information the tcp information and that now there is http protocol information as well within our get request this is our get request to get a web page if i go to the next screenshot the last screenshot shows an icmp version 6 neighbor solicitation message if we look up in the window at the highlighted packet here in packet number one we'll see that the source address this time is not a global unicast ipv6 address but a link local address we can tell that from the fe 80 here we can also see that this link local address used eui 64 to resolve the interface identification portion of the address we can tell that by the fffe within the address the destination address is an ff02 ipv6 address indicating that this is a multicast packet if we look over the protocol we see that it's icmp version 6. and then information about the packet tells us that this is a neighbor solicitation message for the same device that we were contacting in the earlier screenshots the function of this packet essentially is similar to an arp request in ipv4 we need to discover the link local address of this device so we send out an icmp version 6 neighbor solicitation message multicasted and we're hoping to get back a link local address from this neighbor if we look down in the expanded details window we can see the version is six traffic class flow label payload length which is the tire length of the packet the next header field which is like the protocol field in ipv4 indicating a 58 that this is an icmp version 6 message in the data portion of the packet the hop limit 255 hops this is similar to the ttl field and then the source link local address and the destination multicast ipv6 address at the bottom below the ipv6 information we can see that there's an expandable area specific to the internet control message protocol version six all right again hopefully that's a good starting point for you to understand each field and i like how he goes to wireshark to actually show you the actual packet and that packet information you will like i said do a lab on that here in this course um and you'll be able to see that for yourself that's why we installed fireshark a little bit ago all right here we go keep going how does a host route how does a host how a host routes okay uh how a router uh routes uh information from a host maybe that would be a better way to save that uh because ultimately we take uh a a packet of information a frame of information from pc1 and if it's if it's within a network it can transmit right here within the network it can it can transmit to itself do using something called a loop local now loop will go away hey um up it's i've lost a word here sometimes it happens when you're in front of a camera yeah that was uh that's a loopback uh when it talks to itself uh that would be uh one two seven zero zero one or two or three and you can have several different loopbacks they can also talk uh to local hosts within the same lan so it never leaves talking to the switch here and then it can also obviously talk to the internet things out on the web such as this remote host the source that source device determines whether the destination is local or remote the method of this determination well if it's ipv4 the source uses its own ip address and subnet mask along with the destination ip address and subnet mask to determine if it's on the same network ipv6 the source uses the network address and the prefix usually 64. don't worry we'll get there advertised by a local router local traffic is dumped out the host interface to be handled by an intermediary device switch router etc remote traffic is forwarded directly to the default gateway and i don't like the way it says uh forwarded directly to the default gateway it's almost like somehow they think there's a way around the switch no it it ultimately makes it through the switch to the router so don't there's no magic here it is uh still uh passed through our switch we learned that last week we spent a lot of time understanding how packets are uh how our requests are made for example to be able to get uh the address of the router to be able to send information to uh uh another address on the local network or the router the def uh the the gateway of last return the edge router heading out to the internet a router or layer 3 switch can be a default gateway uh the features of a default gateway abbreviated dgw one of my least favorite acronyms it must have an ip address in the same range the same network as the rest of the land it cannot accept data from the land and is capable of forwarding traffic uh it cannot accept data from the land and is capable of floating traffic off of the land i think there's a typo there it can accept data oh my goodness how many times can i read that and have it read wrong it can accept data from the land from other hosts on the network and it can send information off and then it can also route to other networks if a device has no default gateway or a bad default incorrectly configured gateway its traffic will not be able to leave the lan and those packets will be dropped post routes to a default gateway the host will know the default gateway either statically or through dhcp in an ipv4 network uh the uh statically will learn in this presentation in just a few slides it might be in the next presentation but i think it's in this presentation ipv4 sends the default gateway through a router um solicitation rs or that's ipv6 ipv6 uses a router solicitation message to send information to the the default gateway and it can be configured manually default gateway is his static route which the default gateway uh is a static route which will be a last resort route in the routing table at the bottom and all devices on the lan will need a default gateway off the router to the router if they intend to send traffic outside of the network all right on a windows you can see the routing table by typing netstat dash r uh remember a router is just a special type of pc so you can us it works in the same way you just get to it a little bit different with this netset dash r and here in this routing table you can see that you have some multicast routes you have a all nodes broadcast and you have a couple individual addresses here for example you know about uh this one device that's on that network anyways don't worry too much about uh the pc's routing table it's pretty easy to understand uh where it's going to uh and it's the ip address and the uh network map that netmask the the subnet uh the gateway and then the interface that you are going to exit your pc and then um the metric is the cost of the the traveling between the source and the destination you learn more about that when we get to to cisco 2 and then ospf afterwards we don't really learn too much about the cost of inner routing inner router routing in this course uh the introduction introduction to routing okay so how does a router forward packets well uh the uh receive the packet receives a frame of the the pc sends a frame and it sends to in this case it's sending to uh the ip address 10.1.10 10.1.1.10 and um that's the destination and uh for multiple packets they're sent uh through the router now that that that's this pc over here uh so the right in this case the uh the pc knows the destination p pc's address uh the router knows it this is router ones table you can see that it already has router one uh router one i'm sorry that's this packet this router ones routing table it doesn't look like this at all but we'll see that soon enough uh so this is the routing table it says yeah to get to this network you go through that network a router two and off it goes right uh to get uh to the default gateway um let's see and it also knows how to get to the ten network here uh it's directly connected to g g0 etc it's uh on interface g00 it's actually connected to the switch it knows how to get to this network that's on this interface and it also has a static route that goes we call that quad zeros that means anything that is not uh included above any routes that are not in the routing table if you wanted to go to google 8.8.8.8 for example you would it would this default route would pick it up and it would be sent to the default gateway which is right here r2 and then r2 would handle it and say hey do i know where google is and if it does it would send it directly there if it doesn't it would send it to its default gateway and that's pretty much what that writing says there there are three types of routes in a routing table in a router's routing table there are directly connected routes so if you take a switch and you plug it in the router will consider that a directly connected network and it will configure it automatically and any host that is on that network will be able to talk to the router once it has the default gateway configured it can a router can route to remote destinations and that is either manually or dynamically manually it uses a static route we learn that on the next slide i think and then it can also learn about destination routes dynamically using eigrp ospf rip there are several different protocols that routers talk to each other on and then you can also have a default route that forwards all traffic to a specified uh in a specified direction when there is not a match to in the routing table so all they're saying with this video is we have uh the router can uh is directly connected to this this router is directly connected to this network and that network so you'll have a c in the routing table that indicates directly connected and you'll also have an l for the actual interfaces we'll see that in a few minutes l2 is connected to this network and this network and it will have directly connected um entries in its routing table for for the g00 on 10.1.2.1 and 209.165.200.226 which is on g001 that'll be on router 2 and of course it also have this link as well now router 2 can learn about this network through r1 uh you can either put it in a default route well you want to go default route that way but you yeah i'm sorry not the default um you can put a default route that way but you would probably use one of these guys this is called a static route and basically the router you have a static route telling nr2 telling our are sorry nr1 no no in r2 you'd have a static graph saying to get to this network you need to go through this network r1 r1 would have a default route they're showing it here static route that you put in not a default route sorry static route that you put in saying hey to get to this network the way you do it is you go this is the command ip route by the way ipv6 uh is ipv6 route and then the network you're going to the network that networks subnet and then the next hop ip address so that's this address right here 226 we're going the other way it would be um from r2 to r1 to get to this network it would be iprout uh to get to five five two 192.168.10.0.255.25525 five how you would use the next hop this router would use the next top ip address here uh 20209 165 200.225 i just showed you going both ways this router it shows it here going here and this router uh i just told you what it would be going that way just to give you another example uh now uh the um if you have a manually configured static route which is fine for small networks that don't change you have a problem when a router a line breaks because if this line breaks the static route says go this way the router will not can reconfigure it will not configure the router the information to go this way because you have a static route saying how to get to the next network it's not going to go this way now there are ways to add floating routes which is a second static route that would allow you to go this way and it would only become enabled uh if the the primary line goes down you wouldn't even see it in the routing table it would be there and it would show up only if the first line goes down all right dynamic routing dynamic routes route automatically they discover remote networks they're made they maintain up-to-date information they choose the best path to the destination and if there's a problem they find the new best path uh when or when there's a topology change if there's a break in the line or topology change it will find a new best path remember or i should say the router only keeps the best path if there are two or three or five different routes to get from one place to another it figures out the router if it's using it dynamically it figures out which is the best route remember that thing i talked about metric a little while ago it determines which is the best route if it's rip it's just how many how many tops along the way but if it's eigrp or ospf which are not discussed in this class uh but if it's a one of those routing tables it uses a metric uh and a metric is just saying a highway uh uh is a uh faster connection than a dirt road right that's metric dynamic route routing can also share static default routes with other routers so just because you configured a router one to have a static route to r2 if you if you set up ospf er grp between these three routers uh r1 will share the static route with other routers so they'll end it they'll have about a path to get to the network as well and i get a video for you it is eight five five a router uses information in its routing table to forward packets a routing table displays entries listing all the networks that a router is aware of and the bank account to reach them it is very important to be able to read and understand the entries in a routing table in this demonstration we'll take a look at a router's ipv4 routing table in detail but first let's examine the network topology it actually consists of five separate networks or subnets there are four lands two connected to each router and one wan connection between the two routers if we look at r1 it has three directly connected networks network one 192.168.1.1 zero connected to interface g zero slash zero slash zero network 192.168.2.0 connected to interface g zero slash zero slash one and network 209.165.200.224 connected to interface s zero slash one slash zero r2's lands are not directly connected to r1 therefore they are considered to be remote networks as far as r1 is concerned in order to forward data to remote networks a router needs to learn about them first through the use of static or dynamic routing in this example r1 has learned about them through the dynamic routing protocol ospf which has been configured on both routers to view the ipv4 routing table i'll click on router r1 and from the cli tab i'll press enter to connect to the command line from here i'll type enable and press enter to enter privilege exec mode and from here i'll issue the show iprout command to view the routing table if i press the spacebar i will see the full output at the top of the output are letter codes that indicate how each network aka route was learned this is referred to as the route source underneath that we can see the routing table entries these represent all the networks that our one knows about and the best way to reach them let's further examine some individual entries first let's look at the entry for network 192.168. the letter c in front of the entry is the route source and indicates that the source of this route is a directly connected network and you can also see that in the entry here gigabit ethernet 0 0 0 is the interface to which that network is connected now let's look at the entry for network 10.1.1.0 this entry has a route source of o which indicates that it was learned via ospf routing after the network address you can see two numbers in brackets that are used by the router to help determine the best path to the network the first number is the administrative distance which indicates the trustworthiness or preference rating of one route over another next is the metric another value used by the router to help determine the best path the metric may be calculated using hop count bandwidth or some other factor this network is reachable via this next hop address which represents an interface on router r2 this is a time stamp that tells us how long ago this router last received an update on this route and serial zero slash one slash zero is the exit interface on r1 through which to send the packets for the purposes of this demonstration you can ignore any entries that do not list a route source at the beginning these are basically headings also note that for each directly connected network you have an entry below it with a route source of l l refers to a local route and basically this is the ip address of the interface to which that network is connected this routing table shows that r1 is aware of all five networks present in the topology it has three directly connected networks it has two networks that are remote and were learned through ospf routing and lastly if you look at the last entry in the table you will see a statically configured default route this manually configured route can be used to forward any packets that don't have a specific entry in the routing table the purpose of a default static route is so that the router will not drop any packets these are just some of the basics of an ipv4 routing tablet okay and so let's get to the actual routing table and see if i can add anything to her video um i i just want uh i i love having this table for the first couple years i taught the course um i really relied on this table to know what each element was and so when you when you type in show iprout it will show you the table that helps you to understand what all these letters mean on the left hand side once you do it for a while then you know what a c in an l and an o and a and uh and what um i'm looking for uh what they all mean a d uh e i g or psd anyways so uh when we get down below uh i think you can see here is a static route that's been configured and it's the default gateway the gateway last resort and below that you have something learned by ospf two directed directly connected networks this is r1 so we have two directly connected networks and we also know about this network uh through ospf the router is basically chatter r1 and r2 talk so we don't have a directly connected we don't have a static route and it's not directly connected how are we going to learn about this network we're going to learn it through ospf you can also learn it through eigrp which is um taught in cisco three all right and uh yeah and again it wasn't clear when i first took the course way back when uh the uh c this is the network address right so the 10 network immediately after the c you'll find an l if you do a routing table show iprout on a router that doesn't have l's that means you're using uh the operating ios 12 or earlier ios 15 added the ls the local connection so this is the ip address for that network this is the ip address for that network hopefully that helps and that is the end of this very first president well module 8's presentation i have another one to go through this evening but hopefully that was informational uh and uh we are taking it in bites right you can't eat the elephant unless you do it bite by bite right that's what we're talking about with cisco actually all of these courses build one on top of another so all of these concepts are online and cisco we're learning in cisco 2 we revisit in cisco 3 i mean cisco 2 and then we visit them again in cisco 3 and we build and we build and we build until we understand the entire animal and not just a toenail so my name is don lafond professor don it's been my pleasure to teach you today if you have any questions and you're live with me now just wait a moment and you can ask if you are watching this inside of our netiquette classroom please ask your questions in the help discussion forum and if you are watching this on youtube please rate and review and i um please rate and review and ask questions down below and i will visit occasionally and take a look and see if anybody's chatting and i love to participate uh with my students so you can you can count on that thank you very much for coming i'll see you later bye now