By default apache tomcat runs on port 8080.
Instead of running tomcat on port 8080, we can also run it on port 80.
Run following command as root user in machine you want make tomcat to work on port 80.
$ iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT --to-destination :8080
Above command is using Port Forwarding technique, where all requests coming to port 80 on eth0 are forwarded to port 8080.
Advantage of port forwarding technique is security. Since port numbers 0 to 1023 are privileged ports its not suggested to give direct access to no root users.
Now test tomcat on your machine with ip address or localhost
Either of the above command will get the response from tomcat ROOT webapp.