56
LOC REM VNC 100 LNK PWR
IP
Access Switch Plus
Internet
Web server
Ports
If you accept the analogy of
IP addresses
being rather like telephone numbers,
then think of ports as extension numbers. In a company of any size, you
generally wouldn’t expect the accounts department to share the same telephone
with the technical department. Although their calls may all be related to the
same company, they concern very different aspects of that company.
It is the same with IP network connections. Although you have only one network
link into your computer and only one IP address (phone number), you are
probably performing many different tasks through that one link, often at the
same time. Thus, when you browse the web your outgoing requests and the
incoming information are all channelled through port 80. When you send an
email, it travels through port 25 and when you transfer files you are, without
knowing it, using port 20.
At the “border crossing” between the wider Internet and every local network
attached to it, there is a router that is usually combined with a firewall. One of
its main tasks is to direct incoming traffic to the correct place within its local
network. A key piece of information to help it do this is the port number:
Security issues with ports
The settings of port numbers become important when the CPU IP is situated
behind a network firewall. In order for a remote VNC viewer or web browser
to make contact with your CPU IP, it is necessary for the firewall to allow
communication through a particular numbered port to occur.
One specific function of firewalls is to restrict access to ports in order to prevent
malicious attackers using them as a route into your network. Every new port
that is opened offers a new possibility for hackers and so the number of
accessible ports is purposefully kept to a minimum. In such cases, it may be
advantageous to change one or both CPU IP ports to use the same number. The
other alternative is to place the CPU IP unit outside the firewall and take full
advantage of its secure operation features – see
Networking issues
for details.
IMPORTANT: The correct configuration of routers and firewalls requires advanced
networking skills and intimate knowledge of the particular network. LINDY
cannot provide specific advice on how to configure your network devices and
strongly recommend that such tasks are carried out by a qualified professional.
CPU IP has the local IP
address: 192.168.0.3
User with VNC viewer accesses
IP address: 129.7.1.10 (this
automatically uses port 5900).
Router/firewall address: 129.7.1.10
Router is programmed to send port 5900
VNC traffic to local address 192.168.0.3
and port 80 web traffic to local address
192.168.0.42
User accesses the company
website at: 129.7.1.10 (this
automatically uses port 80).
Web server has the local
IP address: 192.168.0.42
