Hi,
i have a index page in /var/www/html, from the local machine i can open the page as localhost or the machine name even the firewall is on or off, But when i type the ip of the machine in another LAN pc it automautically navigate to another folder say like 192.168.1.1 enter -> 192.168.1.1/newpage and gives out 404 error when the firewall is up but i get the page when the firewall is down what do i do?

Adjust your firewall to account for the web traffic. man iptables.

You need to allow inbound traffic on ports 80 (http) and 443 (https)

Something like this in your iptables config script:
Then you might also need to allow the established connections to go back out:

Note the -I flags are inserting the rules at the beginning of the rule chain. This is to make sure they will actually be applied before some other rejection rules kick in.

Now depending on your setup you may want to use the -A flag (append) and place the rules at a specific location in your overall config.

These are just examples that may open holes that are larger than you want them to be. If you post your current config and provide more input on what traffic you want to be blocked more detailed advice could be given...