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What is a default gateway

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:

Source	192.168.1.1	11000000 10101000 00000001 00000001
Destination	192.168.1.2	11000000 10101000 00000001 00000010
Subnet mask	255.255.255.0	11111111 11111111 11111111 00000000

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:

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Tags: IP Routing

Forum Replies

ReneMolenaar says:

Hi Shinu,

It will depend on the host but Windows for example uses a metric to determine which default gateway to pick.

Try a “route print” on a Windows machine when it’s connected with two network interfaces and has default gateways. You can see which default gateway it will prefer.

Rene
ReneMolenaar says:

Hi Miguel,

In the ARP table you only find MAC addresses of devices in the same subnet. When your PC wants to reach something that it not in the same subnet then it will send it to the default gateway. You will only find one entry in the ARP table for everything that is not in your own subnet: the router MAC and IP address, that’s it.

Rene
svvdprasad says:

Hi Rene,

I was reading your lessons since one month onwards thanks for great explanation…

I have one small query

HOST — SWITCH — HOST
10.10.10.2 — SWITCH ---- 20.20.20.2 / 24

will ping work for above scenario … My answer would be No … Since we can’t add the default gateway is that correct answer…

Tried this scenario on GNS,while connecting the host to switch via Ethernet cable it’s throwing error so i thought we could not able to configure Default gateway
it’s throwing an error.

Please help me in understanding this
mrleeiian says:

Hi Rene/Moderators,

With regards to this questions and answers
"
Hi Chris,

IP route is used on a router to enter something in its routing table. The effect will be the same…

Devices like switches don’t build a routing table so that’s when you need to use the ip default-gateway command. Also, on a router you can use it if you disable the routing table with “no ip routing”.

Rene
"

Am trying to fix some problems on L3 switches with ip routing enabled, which have eg.

ip default-gateway 10.10.10.10
and
ip route 0.0.0.0 0.0.0.0 20.20.20.20

Since ip routing is enabled
... Continue reading in our forum
lagapides says:

Hello Ian.

This is a very good question. This often causes confusion.

The ip default-gateway command is used to allow the switch itself to communication with devices outside its subnet. If you have an SVI configured, say interface vlan 10 with an IP address of 10.10.10.5/24, in order for this interface to communicate with the administrator’s PC on another subnet for telnet or SSH connectivity, then it requires a default gateway. You would enter the command ip default-gateway 10.10.10.1. This is similar to the default gateway you configure on a PC. Note tha
... Continue reading in our forum

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