You probably have something listening on port 80 for all IPs.
eg:
$ netstat -an
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN
...
in this case ssh is listening on port 22 for all IPs.
if you see 0.0.0.0:80 that probably means apache has
taken port 80...
