Question:
Does your web host support the if-Modified-Since HTTP header?
Answer:
If your web host is GlowHost, then the answer is yes
The if-Modified-Since HTTP header is recommended by several search engines so that their bots know if they should come through your site and re-index it, or simply cache their index and keep it the same. This is good for you because you can save CPU and bandwidth by telling the bots not to crawl your web site if the content has not changed.
This is also good for the search engines because they do not need to waste resources re-spidering your site if the content has not changed.
This header is off by default, but it is quite simple to add it to your HTTP headers by simply adding some lines in your .htaccess file(s)
Official usage examples can be found at:
mod_expires - Apache HTTP Server
You can test if your headers are working correctly using this handy little tool:
If Modified Since Header Tool