TCP/IP Port Numbers - A Non-Programmer's Guide to Web Crossing

All TCP/IP services, such as web serving, mail, telnet, FTP and news, provide their services using a particular TCP/IP port number. The port number is what is used to distinguish one TCP/IP service from another at a given IP address.

This way one server machine can provide many different services without conflicts among the incoming and outgoing data.

More than one Web Crossing server can, for example, run on the same server machine, even using the same IP address, so long as the port number being used is different for each server.

There are many TCP/IP services and each has its own recognized, default port number. You can change these numbers to avoid conflicts with existing services on your server machine, to hide services from the public or to make your settings correspond to another server's port number settings.

The default TCP/IP port numbers for Web Crossing services are given in the table below, along with some comments about their use.

It is easy to check your server machine to see if a service is already operating at a given port number. Simply Telnet to that IP address and port number combination and see if there is a response.

For example, to see if POP3 is active on the Unix machine at IP address 210.225.17.13, enter the command

Service	Default Port Number	Notes
Direct Web Service	80	If you are serving pages to the public via the Internet it is usually best to leave this at port 80. If you change the port number, people who are accessing from behind firewalls via proxy servers will not be able to access. If you need to run more than one direct web service (for example, both Apache and Web Crossing) it is best to set up IP Aliases and run both services under distinct IP addresses using port 80. For Intranet use and for hiding your Web Crossing server from the public, it is easy to change the port number. If you are running Web Crossing as a CGI under another web server then the port number is normally set to zero. This indicates to Web Crossing that direct web service is not being provided and all communication with the user's browser goes through the main web server.
SMTP Mail Service	25	The Simple Mail Transport Protocol (SMTP) service is used to send and receive email. If you are running sendmail on a Unix machine and you want to use Web Crossing 4 as your mail server, be sure to disable sendmail before running Web Crossing or you will get error messages in your webx.log file reporting conflicts in the port number.
POP3 Mail Service	110	The Post Office Protocol (POP) service allows mail clients, such as Eudora, Outlook, Netscape Mail, Arena, Sony Post Pet, etc., to access user mailboxes and download mail. Be sure you are not running another POP3 server on your service machine to avoid conflicts in port number.
IMAP Mail Service	143	Internet Message Access Protocol (IMAP) is an alternative to POP3 mail service.
NNTP News Service	119	Network News Transfer Protocol (NNTP) provides a mechanism for sending and receiving newsgroup messages.
Chat Service	3155	The Chat services in Web Crossing are proprietary and use special port numbers.
LDAP Service	389	The Lightweight Directory Access Protocol (LDAP) provides outside clients access to Web Crossing user directory information, such as usernames and email addresses.