First of all have you any idea how data (information) is moving between computers? Of course I am not asking boring rules and technical theory. Let's simplify the whole communication process and parallel it with real life example.

As we know each computer connected to the internet has its own unique IP address. Similar to the PC connected to the Internet, each computer connected to the LAN has unique IP address with one slight difference. That IP is unique, but ONLY within THAT network. You can imagine LAN as a small Internet network In LAN computers identify and communicate with each other using that unique IP addresses.

A portion of the data (information) which is sent from one PC to another is called package. It is like real package (parcel). It has information about sender, receiver, type (protocol), weight (size in bytes) and other useful information.

• Lesson 3.1 Communicator device - router
• Lesson 3.2 What is firewall and how it works

If you are admin of LAN then you are like the owner of the post office with all rights. You can create any crazy rule for your post officers and they must follow them. You can specify the criteria in which the package should be refused and sent back to the sender. As the owner you have unlimited rights within your post office (I hope now you understand how good it is to be and IT guy in the company ). You could use any package option: senders' address, receivers' address, type of package and so on to create your own rule. The rules could be really complicated: you can allow the same sender to send packages to one destination but disallow to another one.

For example:

Sender: Jamie A. Patterson, 1044 Cantebury Drive, Mineola, NY
Receiver: Heather T. Valle, 1106 Trymore Road, Pipestone, MN
Type: Envelope
Rule: PROCESS

Sender: Jamie A. Patterson, 1044 Cantebury Drive, Mineola, NY
Receiver: Michael M. Guzman, 1708 Carolyns Circle, Dallas, TX
Type: Box
Rule: STOP

So now every time you are opening google.com web site form you PC the package is sent to the server (strongest computer in your LAN). We know that package contains at least information about sender, receiver and type. If there will be any rule in firewall that blocks such package it will be immediately stopped and error message will be sent to you. Otherwise your package will be forwarded to the receiver, in our particular example - to the google.com server (74.125.87.99).

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I like these type of explanations.  Confirms and add to what I know about the internet world.  Nice to have when communicating with IT people.