How it works…

In the case of reducing DNS lookup, when you open any web page for the first time, it performs DNS lookups through all unique hostnames that are involved with that web page. When you hit a URL in your browser, it first needs to resolve the address (DNS name) to an IP address. As we know, DNS resolutions are being cached by the browser or the operating system or both. So, if a valid record for the URL is available in the user's browser or OS cache, there is no time delay observed.

All ISPs have their own DNS servers that cache name-IP mappings from authoritative name servers and if the caching DNS server's record has already expired, it should be refreshed again. We will not go much deeper into the DNS mechanism. But it's important to reduce DNS lookups more than any other kind of requests because it will add a more prolonged latency period as any other requests do.

Similarly, in the case of using image maps, imagine you have a website where you have inserted separate images for separate tabular menus instead of just plain text to make the website catchier! For example, Home, Blogs, Forums, Contact Us, and About Us. Now whenever you load the page, it sends five requests, which will surely consume some amount of time and will make the website a bit slower too.

It will be a good idea to merge all such images into one big image and use the image map technique to reduce the number of HTTP requests for those images. We can do it by using area and map tags to make it work like the previous one. It will not only save a few KBs, but also reduce the server request from five to just one.