Most ASA run as routed firewall. They have interfaces that act as a layer 3 hop. This means they have an IP address configured on an interface that act as default gateway for the inside and outside networks. This means inside and outside have different subnets
The reason to have a transparent firewall is to drop it in an existing network and become as L2 device. It still does traffic filtering/inspection. You will have same IP subnet inside and outside. IP address on the firewall configured for management only. In a transparent firewall, there is no layer 3 hop between the protected and unprotected segments.
The management 0/0 interface is a Fast Ethernet interface designed for management traffic only.
To see what mode the firewall on now: #sh firewall
If on routed mode – switch it to transparent – (config)#firewall transparent
Typically, you use e0/0 for outside interface and e1/0 for inside interface. You can have multiple sub interfaces if you plan to host multiple context under one interface. Therefore, you can assign each sub interface to each context.
How to create sub interfaces:
(Conf)#int gigabitethernet 0/0.100
(Conf-subif)#vlan 100
You can use your vlan number to the sub interface
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