Understanding static routing

Understanding the routing process is essential for any professional who wants to work with networks so let's consider the initial scenario below. We are going to talk in this post about static routing, which is the basis for understanding dynamic routing, using protocols such as OSPF.

 

 

 

In the figure above we have three computers "PC1, PC2 and PC3" and we also have three routers "R1, R2 and R3". We will need to define five networks as shown in the image below. Note that interfaces 1 and 2 of each of the routers are also identified.

 

 

 

Now we can define the IPs according to the following information:

PC1 - 192.168.0.10/24

R1 - Interface 1 - 192.168.0.1/24

PC2 - 192.168.1.10/24

R3 - Interface 2 - 192.168.1.1/24

PC3 - 192.168.2.10/24

R2 - Interface 3 - 192.168.2.1/24

 

Defined the networks where the PCs are, we are now going to define the link networks where IPs were saved and that is why the /30 mask was used.

 

R1 - Interface 2 - 192.168.11.1/30

R2 - Interface 1 - 192.168.11.2/30

R2 - Interface 2 - 192.168.10.1/30

R3 - Interface 1 - 192.168.10.2/30

 

It is necessary to be well understood to have all IPs of all interfaces of all equipment in hand. Now for the explanation.

PC1 for example can communicate with any other device that is on your network "192.168.0.0/24" even without a gateway, and this goes for the other PCs also just changing as networks, in case PC1 needs to communicate with a device from another network it will need to forward the packet to someone who knows other networks and that's where the gateway "192.168.0.1/24" or the router comes in. If PC1 tries to ping the IP 192.168.11.1 it will succeed because R1 that is your gateway knows this network and will make this communication possible and if PC1 pings the IP 192.168.11.2 it will also succeed because R1 will take care of it to mediate the communication because the R1 knows the networks 192.168.0.0/24 and 192.168.11.0/24 but the communication as it is until now stops there. 

Now for PC1 to ping the IP 192.168.10.1 it will not succeed because R1 does not yet know about the existence of this network and that is where the routes come. In order to have communication, it will be necessary to add a route on R1 as follows: 

R1 asks: 

I want to go to the 192.168.10.0/30 network to whom should I forward this packet? 

 

Then the route should be placed as follows on R1: 

To go to the 192.168.10.0/30 network, send the packet to the IP 192.168.11.2 and then R2 will be in charge of sending the packet to the desired network. 

 

In the system, when listing the routes and after being added the route, something like this would appear in R1:

R1 -> 192.168.10.0/30 via 192.168.11.2

 

So did you get the idea? 

However, to make the communication complete, a route on R2 would be necessary because R2 does not also know the 192.168.0.0/24 network. On R2 after adding the route and listing them it would look something like this:

 

R2 -> 192.168.0.0/24 via 192.168.11.1

 

Because R1 knows the 192.168.0.0/24 network and will be in charge of mediating the communication. 

I hope the idea is clear, below I will put all the routes to have full communication between all devices.

 

R1

R1 -> 192.168.10.0/30 via 192.168.11.2

R1 -> 192.168.1.0/24 via 192.168.11.2

R1 -> 192.168.2.0/24 via 192.168.11.2

R2

R2 -> 192.168.0.0/24 via 192.168.11.1

R2 -> 192.168.1.0/24 via 192.168.10.2

 

R3

R3 -> 192.168.0.0/24 via 192.168.10.1

R3 -> 192.168.11.0/30 via 192.168.10.1

R3 -> 192.168.2.0/24 via 192.168.10.1

 

The ideal to understand with total clarity is to make a diagram on paper and even make a laboratory with the packet tracer, for example, practice to fix this knowledge in the mind. But the idea is to ask the questions, where do I want to go? Who should I send it to? You must point the IP of the neighboring router interface of a network that is known to the router you are working on.

 

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