Here is a basic configuration for a Cisco 2621 using interface FastEthernet0/0 to connect to your ISP, and FastEthernet0/1 to connect to your local network.
Router#sh run Building configuration... Current configuration : 1158 bytes ! version 12.3 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname Router ! boot-start-marker boot-end-marker ! no aaa new-model ip subnet-zero ! ip cef ! interface FastEthernet0/0 description outside interface ip address 100.100.100.1 255.255.255.252 speed 100 full-duplex ! interface Serial0/0 no ip address shutdown no fair-queue ! interface FastEthernet0/1 description inside interface ip address 200.200.200.1 255.255.255.0 speed 100 full-duplex ! ip default-gateway 100.100.100.2 ip http server ip classless ip route 0.0.0.0 0.0.0.0 100.100.100.2 ! line con 0 line aux 0 line vty 0 4 login ! ! end
