Firewall is basically a router connecting two different networks with Access-Rules. Access-Rules involve in specifying the Source PC, Destination PC, Protocol (e.g. HTTP, TCP), Socket Port No. Such Tabular rules are created in Access-Groups and applied either on the outside or inside ports of the Firewall. In this way the Firewall will allow only Filtered Traffic as a security measure.
NAT – Network Address Translation may be required to allow a inside PC appear in the external network as if it is a part of their network. That is the Inside IP is translated to outside IP. This will help in more security as the outside user will never know the actual inside IP.
Please note in most modern Firewalls, NATTING is Compulsory, without which the firewall may not work.
Also Gateway address MUST BE mentioned in the Inside Network PC. Outside network PC may / may not have Gateway address.