Is each firewall interface IP is considered as external IP?.
Do we need to scan each interface IP??.
What is an internal scan? What are the elements for internal, how we do it?.
An external vulnerability scan must scan all machines that are in scope for PCI-DSS that have public ips from outside the firewall (eg the internet). This must be done by a qualified party called an ASV. The PCI Council maintains a list of ASVs on their website.
The internal vulnerability scan must scan all machines that are in scope for PCI-DSS from inside the firewall (eg local network). You may perform this scan yourself.