## **What the heck is DNS-Prefetching anyways?**

DNS-Prefetching is how you resolve Domain Names before the resources are requested. Resolving domain names here means resolving a website's IP address.

**What happens if you don't do this?**

If you don't leverage DNS-Prefetching then that can add several extra seconds for the resource to load & that can severely impact the core web vitals performance.

## **How to identify 3rd Party Domains being used by the page?**

Several tools can help with this.

1. Chrome DevTools can reveal what all 3rd party domains are being used by the page.

		
																										
		2. GTmetrix

3. WebPageTest

These are the 3 tools that will reveal the 3rd party domains that the page is utilizing for loading resources on the page.

## **How to identify the DNS Lookup time it takes currently?**

The same 3 tools will reveal the DNS Lookup time that is being consumed currently.

Here is an example of how you can identify that from Chrome DevTools

		
																										
		## **Coming back to the main goal of this post, should you DNS-Prefetch all 3rd party domains?**

The answer is no, even if you Google it you will find various resources stating that you shouldn't. The reason is quite simple.

If in the first load x number of 3rd party domains aren't going to get requested then DNS-Prefetching them will instead increase the time defeating the purpose of why we are doing this.

But.. I wanted to test this & that's precisely what I did. And results seem conclusive that you shouldn't DNS-Prefetch all 3rd Party Domains.

But don't take my word for it, have a look at these screenshots that represents the source of truth.

		
																										
		As you can see in the screenshot above, the DNS Lookup time drastically reduced when I started Prefetching only those domains that were being used in the first load instead of DNS-Prefetching all the 3rd party domains that the page utilizes.

But wait a minute How did I identify 3rd party domains being used in the first load?

I scanned the page on WebPageTest where it shows a waterfall chart for 1st Run, 2nd Run & So on.

From those Waterfall chart you can identify those 3rd party domains and select those for which DNS Lookup is taking place.