U misunderstood. I wanted apache/tomcat to listen both to ports 80 and 81. 80 when I am accessing zimbra at home. 81 when I am accessing externally since 80 is already occupied.
I cannot translate with my router both port and IP address, just IP address. It is really annoying and I might just go ahead and replace my netgear router with a real machine I use port forwarding on. But for now, if something comes into my network on port 80, I can forward to the web server.
If something comes on 81, I can forward to zimbra. But I can't change the port # from 81 to 80 on the fly. Thus i need zimbra on the forward to be able to respond to http requests on 81.