Perimeter Router, Internal Router and Firewall

Perimeter router is router which is used to provide a connection to the untrusted network also known as the internet. It is also used to provide a local area network (LAN) connection among the trusted network which is the internal network inside the organisation.

Internal router is used as an approach if the perimeter router goes down. There will still be connection even when the perimeter router is down thanks to the internal router. An example would be if there is a denial of service attack on the perimeter router, the network will go down.  Thus, the internal router is there to act as another form of "network" so that an organisation network will not fall.

A firewall is a device or set of devices designed to permit or deny network transmissions based on a set of rules and is frequently used to protect networks from unauthorized access while permitting legitimate communications to pass. Many personal computer operating systems include software-based firewalls to protect against threats from the public Internet. Many routers that pass data between networks contain firewall components and, conversely, many firewalls can perform basic routing functions. Firewalls can act as a packet filters that prevent packets to pass through the firewall unless they match the established rule set.