routing

Communication across two different networks
Now that you understand how systems within a single network communicate through switches,
let's explore how a device from one network can talk to another device in a different network.

Here comes another fundamental feature of Networking, i.e, Layer-3 (L3) Routing.
As discussed in our previous section, we need a "Switch" for devices in the same network to communicate with each other.


Now, for a device in one network to communicate with a device in another network, we need another network equipment known as "Router". The router connects the switches of two different networks as shown in below picture.


We need router also to connect to internet. Here connecting to internet is nothing but connecting to another Router which is handled by our (Internet Service Providers)ISPs.

The data from one device gets routed to another device in a different network based on the information that is gathered in the following three tables:

• Address Resolution Protocol (ARP) table,
• MAC table and
• Route table

All the hosts in a network and router uses both ARP and Route tables, where as a switch uses only MAC table in this routing process as shown in below picture.

In this section, let us understand, how the required information gets gathered in these tables and how that information helps in L3 routing process.

In the previous Switching section, we have learnt about the MAC table, let us understand ARP and Route tables in the section. Prior to it, we need to understand about Gateways in Networking.

Here we go !!!

Route Table
Route tables are used by routers and computers to make routing decisions. They determine how to forward packets based on the destination IP addresses.

Technically, a route table is a data structure that stores information about network destinations and the paths (or next hops or next routers) that should be taken to reach those destinations.

In the above picture, for the device 192.0.10.10 to communicate with the device 192.0.20.20, we need a Route table at source Device and an other Route table at the Router.

Route table of the source device:
When a device is initially configured for network access, it knows about its Gateway either through manual configuration or through DHCP.
So there exists a route on Device's (192.0.10.10) Route table as follows:

We use "route -n" command to view the Route table. Th above Route table shows a default route (0.0.0.0) on the device 192.0.10.10, that directs all traffic (not matching any other specific route) to the gateway 192.0.10.1 through the device's network interface eth0.

Route table of the Router:
For data to pass from Network A to Network B and vice-versa, there needs two routes to be added in the Route table of the Router. These routes can be added using "ip route add" command as follows:

ip route add <Destination-Network> via <Gateway-of-the-Destination-Network>

NOTE: Point to notice, its the Gateway-of-the-Destination-Network, which means, if you want to send data to Host - 192.0.20.20, then its Destination-Network is 192.0.20.0/24 and its Gateway is 192.0.20.1.

This command adds a route in Router, stating that any traffic destined for the Network B - 192.0.20.0/24 should be forwarded to its gateway - 192.0.20.1.
As you can see in the picture, 192.0.20.1 is the IP address assigned to "eth2" ethernet interface of the router. And this 192.0.20.1 IP is the gateway for 192.0.20.0/24 network.

This command adds a route in Router, stating that any traffic destined for Network A - 192.0.10.0/24 should be forwarded to its gateway - 192.0.10.1.
As you can see in the picture, 192.0.10.1 is the IP address assigned to "eth1" ethernet interface of the router. And this 192.0.10.1 IP is the gateway for 192.0.10.0/24 network.

NOTE: If these routes are not available in router, then all the traffic that reaches it, will be routed to internet through its "eth0" interface (as it is default route) and the communication between the two Networks A and B will never be established.

Address Resolution Protocol (ARP)

From Route tables, we understood that to hosts knows the IP of Router, to which it needs to send its data packets. But knowing IP address of the Router alone is not sufficient for a host to send data.
It also needs to know the MAC address of the Gateway ethernet interface.

Here comes the Address Resolution Protocol, using which any device in a network, knows the MAC address of its Default Gateway (i.e, Router's interface).

Suppose 192.0.10.1 is Gateway (Router's IP) of a network, then device 192.0.10.10 obtains the MAC address of its Router's interface using ARP.

The above output shows the ARP table in host 192.0.10.10 where,

• 192.0.10.1 is the IP address of Router's interface (which is also the Default Gateway of that network)
• "00:0e:c6:ba:67:c0" is the MAC address of that Router's interface
• "enx00e04cb7f5c0" is the ethernet interface of the host.

Layer-3 Routing process
Once all the required information is available in ARP, MAC and Route tables as shown in the below image, then the routing process can be initiated, where two hosts from different networks can communicate with each other.

Let us understand the steps involved in for a data packet to move from one device to another device in a different network:

1. Ethernet Frame Creation (by Device): Device forms a data packet with the router's MAC address as the destination. As device is connected to the switch through a cable, this data packet will be forwarded to switch.
2. Switch's role: When the device sends data packets addressed to the router, the switch, using its MAC address table, determines the port to which the router is connected and forwards the packets to that port.
3. Router Decision: The router, upon receiving the packets from the switch, processes them based on the destination IP address. It makes routing decisions and forwards the packets towards their final destination, which may be next-hop router or the destination network.

This is how data moves across different networks using the information that is available in corresponding ARP, MAC and Route tables.