We have a Cisco Router 831 and it's working fine. I have a question regarding the difference between what I commonly think of as 'port forwarding' versus a simple 'permit' applied within an access-list to an interface, ie: 'access-list xxx permit tcp any any eq telnet'.
With port-forwarding a packet arrives at the WAN interface and gets forwarded to a specific machine address on the internal LAN. Let's say it's SMTP port 25 on TCP being forwarded to an e-mail server on the inside LAN. Besides a 'permit' for SMTP against the WAN interface how does the actual forwarding take place? We set this port-forwarding on via SDM but are just getting familiar now with the SHOW RUN and command line interface.
For example, if I just go into an access-list that gets applied to the WAN interface and permit SMTP how will it know which internal machine to send to?
Or to put it another way, if you permit certain ports such as SMTP, Telnet, FTP, etc on the WAN interface but don't specifically tell them to go anywhere is there any risk that they can actually arrive at a machine on the internal LAN? (Wouldn't they just stop at the router because they don't know which specific machine on the inside LAN to go to?)
Thanks in advance for help,
Tom