rename your file's extension to SHTML and use the following SSI(Server Side Include) tag:
<!--#echo var="REMOTE_ADDR" -->
If you don't want to rename your HTM or HTML file to SHTML, use the above in combination with HTACCESS:
AddType text/html .shtml .html .htm
AddHandler server-parsed .shtml .html .htm
The above you put in a file called ".htaccess" exactly that name! Do not put anything infront of the period/dot for this is a special file name that requires a leading period/dot. Put the HTACCESS file within the same directory as your HTM file.
Another method is to use PHP. rename your HTM, HTML file to the PHP extension and use the following tag:
<?php echo $_SERVER['REMOTE_ADDR']; ?>
Just insert those tags within your P taqs as if they were actual HTML tags.