According to best practices, what is the proper placement of a firewall?

Firewalls should always be placed at key security boundaries, which can be the Internet and your internal network. However, proper placement is not exclusive to the boundaries of the Internet and internal networks. For example, it could be placed between two internal networks, such as R&D and guest networks. The demilitarized zone (DMZ) is a segment of a firewall where Internet-facing services are placed. Firewalls are normally not placed only between the DMZ and the Internet because most networks have an internal network.