I have a multi-homed network (5 subnets). Can I use an ASA-5505 to protect them? I know it has 8 FE ports but can all of them be configured with different security levels?
Ok -- let me see if I understand it better:
You have a linux box that is acting as the gateway for 5 different subnets, coming in from 4 seperate links from a pair of switches. You want to replace the linux box with the 5505.
The 5505 can definitely handle 5 subnets, and you will have the option of putting each subnet into the ASA on multiple links, or bringing them in on one link via an 802.1q trunk. Either way, you will have to define the 5 VLAN's and 5 VLAN interfaces on your 5505 and have your hosts point to those interfaces for the gateway. The 5505 (or any other model) will not support configuring multiple IP's on one interface, so you will have to have a seperate VLAN interface/VLAN for each address range.
How many total hosts do you have in this setup? How do you connect up to the internet with your two /24 ranges? If you plan on having a larger number of hosts, I would definitely consider putting in a L3 switch to handle all your routing, and keeping the 5505 to handle the firewalling.