Lifted from routerfreak.com, this is a basic configuration for a Cisco ASA 5505
Step1: Configure the internal interface vlan
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ASA5505(config)# interface Vlan 1
ASA5505(config-if)# nameif inside
ASA5505(config-if)# security-level 100
ASA5505(config-if)# ip address 192.168.1.1 255.255.255.0
ASA5505(config-if)# no shut
Step 2: Configure the external interface vlan (connected to Internet)
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ASA5505(config)# interface Vlan 2
ASA5505(config-if)# nameif outside
ASA5505(config-if)# security-level 0
ASA5505(config-if)# ip address 200.200.200.1 255.255.255.0
ASA5505(config-if)# no shut
Step 3: Assign Ethernet 0/0 to Vlan 2
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ASA5505(config)# interface Ethernet0/0
ASA5505(config-if)# switchport access vlan 2
ASA5505(config-if)# no shut
Step 4: Enable the rest interfaces with no shut
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ASA5505(config)# interface Ethernet0/1
ASA5505(config-if)# no shut
Do the same for Ethernet0/1 to 0/7.
Step 5: Configure PAT on the outside interface
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ASA5505(config)# global (outside) 1 interface
ASA5505(config)# nat (inside) 1 0.0.0.0 0.0.0.0
Step 6: Configure default route towards the ISP (assume default gateway is 200.200.200.2
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ASA5505(config)# route outside 0.0.0.0 0.0.0.0 200.200.200.2 1