TCP/IP advanced: Ports

by Johannes Helmig [Published on 1 Jan. 2003 / Last Updated on 1 Jan. 2003]

You have a basic understanding of TCP/IP ( IP-address, subnet-mask, Gateway, DNS, DHCP,....),
but now it is time to look at an advanced issue, which has a serious impact on network security,
especially when using TCP/IP to connect to the Internet: Ports.
Let's start by looking in the Control-Panel at "Internet Options" :

Connection a single PC to the Internet
is easy, it becomes more complex when
you connect a network of multiple PCs
via a single connection to the Internet.
In such cases, you will need a Proxy,
and you need to configure on the
tab: Connections the "Lan Settings",
where you need to define PORT information.



TCP/IP is a complex protocol, offering multiple services (especially on the Internet), like:
- HTTP (=Web-Browsing)
- FTP (File Transfer Protocol)
- e-mail
- file sharing
- remote support ( like using NetMeeting )
For each of these services, a kind of "sub-dialect" of TCP/IP is used, call : PORT.

Each PORT has a number assigned to it, here a list of the most common PORTS:

PORT: Service: Purpose / usage :
21 FTP FTP: File Transfer Protocol, often used to download software programs and / or
updates from the Internet
23 Telnet Telnet allows to work with a command-line interface on a System,
originally developed for UNIX (so it includes now LINUX systems ! ),
but also available on some NT-systems
25 SMTP used to send e-mail messages to an Internet Mail Server
80 HTTP used to retrieve WWW pages
110 POP3 used to receive e-mail messages from an Internet Mail Server
139 NETBIOS used for Microsoft networking to handle File-Sharing
443 HTTPS used to retrieve SECURE WWW pages


Ports used by NetMeeting :

PORT: Service: Purpose / usage :
389 Internet Locator Server when using a NetMeeting Directory server
522 User Location Server
1503 T.120
1720 H.323 Call setup, negotiate other ports to be used, Audio data
1731 Audio Data
Dynamic H.323 call control (TCP)
Dynamic H.323 streaming [Realtime Transport Protocol (RTP) over
User Datagram Protocol (UDP)]

(more information on ports used by NetMeeting : Knowledge base articleQ158623).

To protect your system against attacks from the Internet using one of these TCP/IP ports, you
should install a Personal Firewall.

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