For example, I used to have a subdomain called blog.viraltech.co.uk that had a few post on it many months ago, but I decided to delete it because it wasn't being updated. However, because there was a few links pointing to that particular subdomain and Google previously knew about it, it was inevitable that Google bots would crawl it and check if there was any updates.
This wasn't a problem with my old web hosting company, and if you typed in blog.viraltech.co.uk or any random subdomain into a web browser, it would just return a "server not found" message, but after migrating to a new web host (ipage.com), several months later I discovered that any random subdomain that I hadn't created or had previously deleted would load and point to my main website rather than return an error message.
I could type absolutely anything such as jimbo-jones-is-greatest.viraltech.co.uk. or w1w2w3w4w5.viraltech.co.uk and they would all point to the main website. Random subdomains in themselves weren't a major issue, but it was subdomains that I had previously registered and used because search engines knew about these and would inevitably start indexing them.
After contacting ipage.com, I was told it was a due to wildcard entry ( * record) set up on my domain and that it had been removed by a technician and my website would take between 1 and 12 hours to resolve itself. I'd never heard of a wildcard entry before, but after a little bit of research, it allows your domain name to be accessed by any random subdomain such as 12345.viraltech.co.uk.
Okay, that sounded absolutely fine, so I waited the recommended time, but the random subdomain problem still persisted. ipage.com hadn't done a particularly good job and only removed the wildcard entry from one of my domains when in fact, I had 4 domains hosted with them. After several more emails to ipage.com support explaining the situation they still couldn't fix the problem so rather waste more time going round and round in circles with their rather unhelpful support I decided to try to fix the problem myself.
It was a fairly quick and simple job. All I had to do was go into "DomainCentral" click on the "More Info" button on the affected domain, and select DNS. After that, you will see Modify: with a drop-down box where you need to choose "A Record".