Permit commands control access by source IP address and port, and destination IP address and port. Users on internal hosts can resolve host names into IP address for both the internal and external systems. Users of external systems cannot access or even see the internal DNS . The internal network is not IP addressable and is completely hidden from the outside. Yet, e-mail, telnet and other authorized applications can access the internal systems.
Internal hosts can use the caching-only DNS to resolve external names.
This allows programs running on the firewall to resolve both internal and external names. No internal address is visible or addressable from the Internet. All out-going addresses are converted to the firewall's address. When specifying IP addresses do not pad the number with leading zeros. If a truncated address ends with a period then the comparison will be made using the character string up to and including the period.