The smallest model firewall, the Cisco ASA 5505, has some differences in hardware and licenses compared to other models ASA devices.
First, no rack. All others are either 1U or larger and can be mounted in a rack.
The second important difference is that it has an 8-port 10/100 2-Port Power over Ethernet. Ethernet port 0 is the default used for the connection to the outside (Internet) zone, and the rest of the interfaces (ports 1 through 7) are used to connect the internal LAN hosts.
The unit has a manufacturing defect out of the box configuration that assigns dynamic IP addresses to hosts on the inside through DHCP. In addition, the default external interface is configured to receive a dynamic IP address from ISP. Since the device interfaces are level 2, are assigned by default to the specified VLAN. Ethernet port 0 is assigned to VLAN2 and ports 1 through 7 are assigned to VLAN1 by default.
Another difference from the ASA 5505 has to do with the license of the device. Base License limits the number of internal hosts (10 or 50) and also allows only 3 VLAN. The Security Plus license permits unlimited internal hosts and up to 20 VLANs.