When a host wants to reach a destination that is outside of its own network, it has to use a default gateway. We use a router or multilayer switch (that’s a switch that can do routing) as a default gateway.

In this lesson I’ll explain how a host knows when to use the default gateway or not and how it works behind the scenes. Let’s start with a simple example:

Above we have two hosts connected to a switch. We only have network 192.168.1.0 with subnet mask 255.255.255.0.

When one host wants to send something to another host then it will check if the destination is inside or outside its own network. When the destination is in the same network then it will use ARP to find the MAC address of the destination and it can send the IP packet. How does the host check if the destination is in the same network? This is done by checking the subnet mask. For example, let’s say that 192.168.1.1 wants to send an IP packet to 192.168.1.2:

The subnet mask tells us which part of the IP address is the network and host part, the host that uses 192.168.1.1 sees that 192.168.1.2 is using the exact same network address and will know that it can use ARP to find the MAC address, create an Ethernet frame, encapsulate the IP packet and send it towards the switch.

Now let’s take a look at an example where we do require a default gateway. Take a look at this picture:[teaser]

Above we have two hosts and two networks:

• 192.168.1.0
• 192.168.2.0

In between is a router that has two interfaces, it has an IP address for each network. The host on the left side will use 192.168.1.254 as its default gateway and the host on the right side will use 192.168.2.254 as its default gateway.

When the left host wants to reach the host on the right, it will check if the destination is in the same network or not:

These two hosts are not in the same network, you can see that the third octet is different. Our host will know that it can’t reach the destination directly and so it will use the default gateway for this. It will be up to the router to get this IP packet to the destination. What happens is that the host will use ARP to find the MAC address of the default gateway.

Once it has the MAC address of the default gateway, it will create an Ethernet frame that has the MAC address of the default gateway in the destination field. The IP packet will remain the same. It will have a source IP address of 192.168.1.1 and a destination IP address of 192.168.2.1.

I hope this lesson has been helpful to understand how hosts determine when to use their default gateway. If you have any questions, feel free to leave a comment.