Installation guide

STEP 6: Understanding network addressing

The first question before you set up your firewall will always be: what address range are you using in your

local network?

Currently, Internet addresses are unique 32 bit numbers, usually displayed for better memorability as four

8-bit numbers separated by full stops (that is anything from 0.0.0.0 up to 255.255.255.255).

Some of these many possible addresses are reserved for special purposes, like local area private networks.

Local Area Network (LAN) addresses

Local area networks (LANs) are supposed to be private networks separated from the public Internet. To

allow the same protocol for the public Internet as for the private network (or intranet), several blocks of

possible addresses have been reserved for private use.

The address reserved for private local area networks are:

• 10.0.0.0 to 10.255.255.255.

• 172.16.0.0 to 172.31.255.255.

• 192.168.0.0 to 192.168.255.255.

These addresses will never be visible to the public Internet, unless the addresses are deliberately translated

in to public addresses first.

The most common way of address translation is called NAT (Network Address Translation). Simple routing

would not work, because nobody from the outside (Internet) can contact any private address without it first

being translated into a public address.

Every single address used in the public Internet must be unique (i.e. the same public address is never used

for two computers anywhere in the world). However, there are an unlimited number of private subnetworks

using the same addresses. But within each private Intranet, the same rules apply as within the public

Internet: every computer must have a unique address.

Which address block to use for a medical practice network

To avoid confusion, this tutorial will refer to network address blocks that always start with 192.168 when

citing examples.

Subnetworks

The third number in the string of numbers (i.e. the digits after the second full stop) will be specific for your

‘subnetwork’.

You can have multiple subnetworks within your practice or practices, but only machines within the same

subnetwork will be able to see each other without a special bridging interface.

For example: Your subnetwork address is 192.168.0.

Now, we can identify up to 256 different network interfaces (and one computer can have multiple

interfaces), namely 192.168.0.0 to 192.168.0.255.

Subnet masks

To allow all these 256 interfaces to see each other, you have to specify a subnet mask.

In the example above, if we want all of the possible 256 computers to see each other, you need to specify

the subnet mask as: 255.255.255.0.