ciscoasa(config-if)# interface ethernet 0/0 ciscoasa(config-if)# switchport access vlan 2 ciscoasa(config-if)# no shutdown ciscoasa(config-if)# interface ethernet 0/1 ciscoasa(config-if)# switchport access vlan 1 ciscoasa(config-if)# no shutdown

**nat** The nat command enables network address translation on the specified interface for the specified subnet.

In this sample, configuration, NAT is enabled on the inside interface for hosts on the 192.168.1.0/24 subnet. The number "1" is the NAT I.D. which will be used by the global command to associate a global address or pool with the inside addresses. (Note: NAT 0 is used to prevent the specified group of addresses from being translated.)

ciscoasa(config)# nat (inside) 1 192.168.1.0 255.255.255.0

**global** The global command works in tandem with the nat command. It identifies the interface (usually outside) through which traffic from nat'ed hosts (usually inside hosts) must flow. It also identifies the global address which nat'ed hosts will use to connect to the outside world.

In the following sample, the hosts associated with NAT I.D. 1 will use the global address 12.3.4.5 on the outside interface.

ciscoasa(config)# global (outside) 1 12.3.4.5

In this additional example of the use of the "global" command, the interface statement tells the firewall that hosts associated with NAT I.D. 1 will use the DHCP-assigned global address on the outside interface.

ciscoasa(config)# global (outside) 1 interface

**route** The route command, in its most basic form, assigns a default route for traffic, typically to an ISP's router. It can also be used in conjunction with access-lists to send specific types of traffic to specific hosts on specific subnets.

In this sample configuration, the route command is used to configure a default route to the ISP's router at 12.3.4.6. The two zeroes before the ISP's router address are shorthand for an IP address of 0.0.0.0 and a mask of 0.0.0.0. The statement outside identifies the interface through which traffic will flow to reach the default route.

ciscoasa(config-if)# route outside 0 0 12.3.4.6

The above commands create a very basic firewall, but frankly, using a sophisticated device such as a Cisco PIX or ASA security appliance to perform such basic firewall functions is overkill. Other commands to use include hostname to identify the firewall, telnet or SSH to allow remote administration, DHCPD commands to allow the firewall to assign IP addresses to inside hosts, and static route and access-list commands to allow internal hosts such as DMZ Web servers or DMZ mail servers to be accessible to Internet hosts.

Copyright (c) 2008 Don R. Crawley

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