Perimeter Router, Internal Router and Firewall

"A simple diagram of a network implementing a perimeter router with firewall capabilities"

So what is a perimeter router? As the name suggests, it has something that got to do with security(DUH!). In this case, a perimeter router is a router(You don't say...) that acts as a border between you and someone else's network.

"The famous 38th parallel that seperates North and South Korea"

Unleash your imagination and pretend that the left side of the line is your home network. The right side of the line will be a stranger's network or the Internet. Now what do you think the soldier on the line will be?
Of course! He is the perimeter router that filters(security checks) out people travelling across from the border.
A perimeter router basically controls data packet traffic at the IP level. It can be employed in two ways - configured without packet filtering rules or with IP filtering router method which selectively allows or blocks data packets based on their port information and the addresses depending on the security policy. Thanks to these two methods, perimeter routers are quite versatile and can be employed in various ways. For example, we can implement a perimeter router to filter out packets going into the network before passing them to the firewall for a higher level of security protection. The way these routers are employed depends on the preferences of the network owner or organization

Internal Router

An internal router is a router(You don't say again...)that makes use of the OSPF(Open Shortest Path First) routing protocol to construct a topology of the network by gathering link state information from available routers as well as maintaining the routing table so that routing updates could be exchanged. One great feature of the internal router is that it can detect link failures(such as router failure) fast and enables the network to recover quickly.

Firewall

"We didn't start the fire~♪"

Now its time to introduce a very important component that exists in (almost)every network out there in the virtual world as well as in every computer, known as the awesome firewall! *Applause, dang it!*

"A common firewall interface which can be commonly found in Windows computers"

A firewall is a component of a network or computer that is programmed permit or restrict network transmissions based on the set of rules the owner or organization implemented. Most firewalls being used nowadays operate on the network and application layer of the OSI model(Does that ring a bell to you?). Just like a perimeter router, a firewall can filter packets and decide if each packet can be allowed to pass or denied to pass based on the rules set in the firewall. There are two types of network layer firewall. Stateful and stateless. Stateful firewalls maintain information on active processes running in the network uses it to speed packet processing. Stateless firewalls operates with less memory and are quick with simple packet filtering
On the application level, firewalls filter out packets that enter to and from an application. Firewalls can restrict improper content (such as *AHEM* adult contents) based on the firewall rules as well as to block harmful network malwares such as worms and viruses such as trojan horses.
We will move on to the next topic once the owner of this blog returns to his computer. In the meanwhile, have a nice day!