CentOS comes with a firewall that blocks most traffic by default (everything except SSH and ICMP.) You need to allow HTTP through the firewall. Two ways of doing this:
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Run system-config-firewall, go Customize, but a "*" beside "WWW (HTTP)" in the list, then Close, then OK, then Yes
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Run these commands from the shell:
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
service iptables save