Ports
All Internet traffic uses ports. Most of the traffic
you see as you "surf" the Internet is coming across port 80,
which is the port for http traffic. There are other ports that are commonly
used. Think of them as lanes on a highway. Traffic comes and goes on
the highway, and it all flows the same directions. Ports differentiate
protocols. Some commonly used ports are:
20 - FTP Data
21 - FTP command
22 - SSH (see also this guide page)
23 - Telnet
25 - Outgoing mail (SMTP)
80 - HTTP
110 - Incoming Mail (POP3)
119 - Usenet (NNTP)
139 - MS NetBIOS
A more complete list can
be found here.
IP Addresses
IP (Internet Protocol) addresses are similar to your home address. Just like
every home in the U.S. has a unique home address, every computer connection
has a specific IP address. An IP address is a string of four numbers,
separated by a period ( . ). These numbers can be in the range 0-255.
An example is 198.69.224.5.
Note that not all computers have "routable" IP addresses.
For example, if you have a cable or DSL connection at home, and you
are running more than one computer through a router, the router will
have an IP address, and it will assign the computers inside the network
a series of "unroutable IP addresses". For example, 192.168.*.*
are unroutable - no computer actually connected directly to the Internet
will have these IP addresses - they are reserved for internal connections.
Firewalls
A firewall is a program or device that sits in between
your computer and implements a set of rules as to what kinds of traffic
can or cannot come into the network. Many people employ firewalls to
protect them from hackers. This is done by blocking traffic on ports
that would likely invite hackers - services that are left on by default
that have security vulnerabilities can be configured to be blocked.
