Original subtitles

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Here’s my local PC

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if I type ip config I can see my PCs IP address

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and I can see the default gateway

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my PC will send traffic to the default gateway

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for any IP addresses not in the 192.168.56.0 subnet.

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so as an example, if I ping google.com

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that traffic is going via my default gateway

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in the same way, I could telnet to a router.

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here’s a Cisco router that I’ve got in my lab.

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And if I type the command sh ip route

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I can see the routing table of the router.

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The router has directly connected networks.

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So C is directly connected networks

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as an example 192.168.56.0/24 is directly connected on this interface F0/0

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so if I ping 192.168.56.1 that traffic is going to be sent out F0/0

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because that network is directly connected to this interface.

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Packets will traffic to 192.168.58.0 will be forwarded out of interface 0/1

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notice this route S and * this is a candidate default route

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in other words this static route is the default route that a router gonna use.

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If I look at the running configuration of this router

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and then include only lines that have IP router in them.

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I can see this command was added to the router

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so I type IP router 0.0.0.0 0.0.0.0 and then 192.168.56.1

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This portion is saying that we are adding a default route

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to the IP routing table of the router

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with the gateway of last resort being 192.168.56.1

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so basically this router will forward traffic to 192.168.56.1

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for any networks not in it's routing table.

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So once again sh ip route

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shows me I have the 3 network 192.168.58, 192.68.56 192.168.97 in the routing table

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but I don’t have as an example a route for google.com.

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Google’s IP address is 216.58.213.100 that’s not in the routing table

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but I was able to ping that address because of this entry in the routing table.

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In other words, the local router forwarded traffic to 192.168.56.1

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to firstly get to the DNS server 8.8.8.8

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and secondly, to ping Google.com we can see that by using the traceroute command.

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So let's trace route to Google.com

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notice the first hop is 192.168.56.1 the local gateway of last resort

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traffic is then sent out into the internet to get to google.com

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so my traffic is being forwarded from router to router

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until it gets to 216.58.213.100

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this stars or * indicates that the router is not replying back

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to the ICMP messages sent to it.

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So ICMP maybe drop as an example

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but the point to take note of is based on the IP routing table

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on this router, the gateway of last resort or the default gateway is 192.168.56.1

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this static route has been added to the routing table

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and I could remove that by typing no ip route

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Selecting the route so now when I type sh ip route

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notice the static route that was their previously has now been removed

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so now if try and ping google.com

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the DNS resolution is failing and we won't be able to ping google.com

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because we can't resolve the DNS name to IP address of Google.com

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in this case, it's re-using the cache entry

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and is sending traffic to 216.58.213.100

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but notice the pings fail, we had a 0 success rate.

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If I add this static route back again

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and type sh ip route static route is back

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and now when we ping Google.com the ping succeeds.

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So static routes are still used quite heavily

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especially on routers at remotes site or small sites.

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Here’s my router at home.

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This is the 192.168.56.1 router

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that’s my Cisco router had as it's default gateway

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and this router will also have a default gateway connecting it to the Internet.

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So I'll log in if I look at the broadband connection

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notice my local router has this IP address

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and the default gateway is set to the IP address, primary DNS and secondary DNS

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are set to these values, so my home broadband router

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or DSL router has its own default gateway.

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This Cisco router is forwarding traffic to the BTDSL router using this static route

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and the BT router is forwarding traffic to this default gateway.

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Every router makes its own decision on where to forward traffic to

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hence following the hop by hop paradigm.