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Default Route

A default route provides a fallback path for network traffic when a router has no specific routing table entry matching a destination address, forwarding packets to a designated next-hop router instead of dropping them.

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About this video

Routers populate their routing tables through two primary mechanisms: directly connected networks are added automatically when an interface is assigned an IP address and brought up, while remote networks are learned either through static configuration or routing protocols and reached via a next-hop address. These entries allow a router to make precise forwarding decisions for known destinations, but they leave a gap when traffic is destined for a network the router has no record of. Rather than dropping those packets, a default route provides a configured fallback that instructs the router to forward any unmatched traffic to a designated next hop. The default route is commonly represented in routing tables as a destination of 0.0.0.0 with a mask of 0.0.0.0, which matches any address and carries the lowest route specificity, ensuring it is only used when no more specific entry applies. In practice, default routes are chained across network boundaries. An internal router may forward unknown traffic to a perimeter or edge router, which in turn holds its own default route pointing toward an internet service provider. This tiered approach means internal devices and routers do not need full knowledge of every reachable network on the internet. On a Windows endpoint, the ipconfig command displays the configured default gateway, and route print shows the corresponding 0.0.0.0 entry in the local routing table, both pointing to the same next-hop router address that handles traffic destined beyond the local subnet.

What you'll learn

What's covered

Network Routing Concepts

Aligned to

Cisco CCNA
3.1 Interpret the components of routing table
3.2 Determine how a router makes a forwarding decision by default
1.10 Verify IP parameters for Client OS
CompTIA Network+
2.1 Explain characteristics of routing technologies
Cisco CCST Networking
4.4 Explain basic routing concepts
5.3 Run basic diagnostic commands and interpret the results
CompTIA A+ Core 2
1.2 Given a scenario, use the appropriate Microsoft command-line tool
CompTIA A+ Core 1
2.8 Given a scenario, use networking tools

Key terms

Default Gateway
The router that a device uses to send traffic to destinations outside its local network.
Router
A network device that forwards data packets between networks based on IP addresses.
IP Address
A numerical label assigned to each device connected to a network that uses the Internet Protocol.
Default Route
A catch-all routing entry represented as 0.0.0.0/0 (IPv4) or ::/0 (IPv6) that a router uses to forward packets when no more specific matching route exists in the routing table. It is typically configured to point toward the next-hop router or ISP connection.
Routing Table
A database stored in a router that lists known network destinations, the metric (cost) to reach them, and the next-hop address or outgoing interface to forward packets toward those destinations. Routers consult this table for every packet they forward.

Transcript

We've already learned about a couple different network types. One is a directly connected network, one that's connected directly into one of the interfaces of the router, and that goes into the routing table. The reason, or when it goes into the routing table, is when we configure the exit interface that goes to that network, which means that we get onto that interface, we configure an IP address, we configure a mask to it. When we bring that interface up and that interface is live, it recognizes I'm attached to this directly connected network, and so that is going to be put into my routing table.

It also could have a route to a remote network, and it has a connection to the remote network through a next hop, through another router, and so that can go into this routing table as well.

What a default route is

Now one thing we haven't talked about is a default route that we can utilize to get to different networks.

So let's say that the packet comes into this router and it does not have any knowledge of this remote network over here, but it still needs to get that information over to it. What we can do is set up a default route onto this router that says, if you don't know where that network is, then what you can do is send it to this next hop. And then when it sends it to this next hop, hopefully this router knows of that network and knows to send it to its next hop. So that's a default route.

A default route is, if you can't find any other networks within your routing table that's associated with a destination IP address, then send it to this next hop.

Now, typically we see that associated with a router that has access to the outside world. So on here, we'd set up a default route that says go to this router if you can't find out where it is that you need to send this. And then this router has a default route set to the outside world, or the internet, or your internet service provider.

Seeing it on a Windows machine

I'm back on this Windows machine here. I'm going to pull up a command prompt and we're going to do an ipconfig and hit enter. What I'm looking for is this Ethernet 2 connection right here. We can see that there's a default gateway that's set on it. So this is saying that this is the router that, if you need to reach a machine that's not on the local network, go to this router right here, 192.168.87.1.

Next what I'm going to do is a route print, and this is what brings up that routing table. So notice that is right here, this gateway is 192.168.87.1. I just want to point out that this is the destination network, and if you recall, this is the everything, the every. So essentially what this is doing, this is the default route, that if there's no other route that it can find in this routing table, it's going to choose this default route right here. So that's how it's going to get to the outside world. So this is the default.

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