I just installed centOS 5.1 last night on a brand new server, installation process was fine.
reboot,
ssh to server from my desktop on the LAN is fine,
check, and httpd is running,

then I try to access the server using web browser from my desktop pc on the lan, the test page of the Apache doesn't show up. And Firefox tells that server can not be reached. (ssh can, 80 is not inaccessible)

on the server, wget 127.0.0.1 is fine, wget does get the test page. So it means Apache is running, but somehow, the server reject all http requests from outside.

everything is default setting, I took a look at httpd.conf, none seems suspecious. What problem could this be ? Anyone out there with similar experience ?

So, are you plugging in the IP address in the browser or the hostname? Can you reach this server by hostname with DNS on your network? Do you have a firewall running on it? nmap is your friend in this situation.

1) Check firewall configuration
2) make sure you can get into other services (you said ssh is OK, so this is OK)
3) nmap
4) post httpd.conf

this time going through I realized that it is the SELINUX caused the problem.

by disabling the selinux, the apache test page is accessible now :-)
by default, selinux only allow SSH access to the server.

I read on howtoforge that selinux cause more problems than benefits and author recommend turn it off. Should I turn it off completely or should I keep it on but customize the settings ?

BTW, this is an absolutely great forum, and you guys are extremely helpful, just want to say thank you.

I set SELinux to warn level.
Once I finished configuring my server, I read the logs, adjusted SELinux and then set it active again.
If you have a often changing environment, then leave it to warn level, or get a dev Virtual machine at home.

I disable it always.

Oops. I totally forgot about that. I use OpenSuSE, usually with Novell AppArmor which comes with an Apache profile...