Installation guide

9
STEP 2: Understanding how the Internet works
Understanding how and why to install a firewall, a basic networking knowledge is required. Here is a
simplified explanation of how the Internet works, using analogies with the phone system. You need to
understand these basics to be able to manage your firewall.
How data is exchanged—TCP/IP and other basics
Nowadays, most networks use a protocol called TCP/IP (Transport Control Protocol/Internet Protocol).
TCP/IP is the collection of communication protocols that manages the exchange of data in what is referred
to as ‘packets’. This is the protocol the Internet uses. Whenever the term networking is used in this tutorial,
assume that the TCP/IP protocol is used.
To participate in a TCP/IP network, you need a network interface. A computer can contain one or more
network interface cards (NIC).
Each NIC must have a unique IP address to participate in a network and to interact with network traffic.
Basic networking principles
Internal versus external networks
Most practices have their own internal phone networks with ‘extensions’. These phones have internal
numbers, usually up to three digits, that people within the same practice can use to dial each other.
The internal network works even if the external phone lines are down, because it doesn’t use the external
phone lines at all.
Likewise, if somebody from outside the practice wants to call a particular extension, usually they cannot do
so directly. Rather they have to ring the receptionist first, and be switched through onto the internal
extension. This is done by pressing a special button on the receptionist’s phone, which instructs your
practice internal phone switch box to connect the external public phone network temporarily, via the
requested phone (extension) in your practice internal network.
Some internal phone systems are installed in a way that allows internal extensions to be dialled directly from
the external public phone system. This is convenient for callers, but can also bypass your receptionist,
exposing you to calls you might not want to receive.
If you want to call outside the local internal phone network, you have to request a ‘line’ by pressing a special
key on your phone so your practice internal phone switch box will establish an outgoing connection for you.
It is similar in the TCP/IP network world:
• telephone company = ISP.
• telephone = network interface.
• private extension phone number = private IP address.
• phone number = public IP address.
• telephone line = network cable.
• PABX/Switchboard = bridge between private and public network.
• receptionist = firewall.