The figure below (taken from Cisco) illustrates the back panel of the Cisco ASA 5505 appliance, showing the network interfaces and other important hardware points.

 Starting from right to left, we have Ethernet0/0 up to Ethernet0/7. The last two Ports 6 and 7 are also Power over Ethernet Ports (PoE), which means that in addition to normal computers, you can also connect IP Phones which will be powered by the firewall PoE ports. The eight network interfaces of the ASA 5505 work only as Layer 2 ports, which is the difference of the 5505 model from the other ASA models. This means that you can not configure a Layer 3 IP address directly on each interface. Also, port Ethernet0/0 is used as the Outside untrusted interface (connecting to Internet), and the rest interfaces 0/1 to 0/7 are used as the trusted Inside interfaces connecting to internal hosts. By default, Vlan 2 is assigned to Ethernet0/0 and the native Vlan 1 is assigned to the rest of the interfaces. Two Switch Vlan Interfaces (SVI) exist by default (Interface Vlan 1 and Interface Vlan 2) which can be used to assign the Layer 3 IP addresses for the Outside interface (Ethernet 0/0) and for the inside zone (Ethernet0/1 to 0/7).

Implementing security in layers is the recommended network security design for protecting information assets. This approach is achieved by segmenting your network into various security zones with an ASA Firewall and applying access policies between them according to their security levels. By default Cisco ASA firewalls come with at least four 10/100/1000 network interfaces which are sometimes not enough. Considering that we need one physical interface dedicated for failover configuration (if used) and that usually one dedicated interface is used for Internet connectivity, then we are left with only two physical interfaces for further network segmentation.

By using the Cisco ASA 4-Port Gigabit Ethernet Security Services Module (4GE SSM) (shown above) you instantly expand your firewall interfaces to a total of three Fast Ethernet and six Gigabit Ethernet ports on the Cisco ASA 5510 Security Plus, and eight Gigabit Ethernet ports and one Fast Ethernet port on Cisco ASA 5520 and 5540 appliances. This gives you plenty of physical interfaces to work with and apply your security design without hardware limitations.