Base URL - How to call the home link

What you need is the virtual directory name of your application/website.
This is a server variable, and it is not possible to get it in HTML alone, you need some server side component, like PHP or ASP.NET.

If you are in the root directory, you can try a relative link.
So try using a . to indicate "from this folder"

<a href="./">Home</a>

Should your question actually concern the opposite direction, do this:

<a href="../">Home</a>

Or use an absolute link:

<a href="/folder">Home</a>

Or use a canonical link

<a href="http://subdomain.domain.com/folder">Home</a>

test case

It appears that if you are going to start your url (a url that depends on a base url that is) with a /, you need to specify that this is from the current directory level. Thus your link should be

<a href="./">Home</a>