include file using html
 

I have been using include file for consistency purpose but I need to use asp file as extension because when I tried using htm as extension, it doesn't work. So, my question is any way to have these 'include' file in htm. Meaning that, for example 3 file have same header, these 3 file just link to one file called inc_header.htm. Hope you guys understand. Thanks in advance for any idea or advice.