Ports: A Purely Mythical Concept

Ports are necessary because a single physical server may be running several (software) services. For instance, a single physical server may be running web services (httpd), secure shell access (sshd), and file transfer services (ftpd).

How is it supposed to know to which service it should direct a request? Port numbers are the answer. Consider web services, which (usually) run on port 80. But you can direct a request to a particular port:

http://localhost:901

This means a request to:

http://my.domain.com

is actually a request to:

http://my.domain.com:80

But it doesn’t have to be. You can configure your httpd server to run on a different port, and when your (physical) server receives a request, it will send that request to the specified port. Why would you do this? You might want to run your web server on a non-standard port to confuse attackers and crackers. But more likely you’ll be forced into a port other than 80 because you have more than one web server running on your computer.

 

Multiple web servers

If, for instance, you’re doing both ASP and Java web development on a Windows PC, you may have the standard Internet Information Services running on port 80 to serve your ASP pages.

But you’ll need a Java application server to run Java code, so you might choose to run Apache with the Jakarta Java application server running under it. Typically, this will force traffic intended for the Java server to port 8080.

This is the most common multi-port web serving scenario, but there are plenty of others.

 

So what is a port?

You know darn well there’s only one (maybe two) ethernet ports on the back of your server box. These aren’t the ports we’re talking about.

The ports we’re talking about are virtual ports. There’s no corresponding physical hardware.

Think of making a telephone call to a friend’s house: you call 222-3333, and someone unfamiliar answers:

“Hello?”

“Hi, can I talk to Jake?”

This is almost exactly the dialog taking place when you request a service on a certain port.

One further detail to notice is that a port is specifically assigned to either TCP or UDP transport protocol (with all that the difference between these protocols brings).

 

Other uses of ports

Ports are extremely valuable for firewall operations. You can open or close any or all of your ports, and block all traffic on them.

One related concept that you may encounter is that of a socket: simply stated, a port plus an IP address equals a socket. In other words, a host (at a certain IP address) offers a service (over a certain port) allowing you to make a connection (a socket).

 

Port Assignments: a partial list

Service	Port/Protocol	Description
chargen	19/tcp	Character Generator
chargen	19/udp	Character Generator
ftp-data	20/tcp	File Transfer [Default Data]
ftp-data	20/udp	File Transfer [Default Data]
ftp	21/tcp	File Transfer [Control]
ftp	21/udp	File Transfer [Control]
ssh	22/tcp	SSH Remote Login Protocol
ssh	22/udp	SSH Remote Login Protocol
telnet	23/tcp	Telnet
telnet	23/udp	Telnet
domain	53/tcp	Domain Name Server
domain	53/udp	Domain Name Server
bootps	67/tcp	Bootstrap Protocol Server
bootps	67/udp	Bootstrap Protocol Server
bootpc	68/tcp	Bootstrap Protocol Client
bootpc	68/udp	Bootstrap Protocol Client
tftp	69/tcp	Trivial File Transfer
tftp	69/udp	Trivial File Transfer
gopher	70/tcp	Gopher
gopher	70/udp	Gopher
finger	79/tcp	Finger
finger	79/udp	Finger
http	80/tcp	World Wide Web HTTP
http	80/udp	World Wide Web HTTP
kerberos	88/tcp	Kerberos
kerberos	88/udp	Kerberos
rtelnet	107/tcp	Remote Telnet Service
rtelnet	107/udp	Remote Telnet Service
pop3	110/tcp	Post Office Protocol – Version 3
pop3	110/udp	Post Office Protocol – Version 3
sunrpc	111/tcp	SUN Remote Procedure Call
sunrpc	111/udp	SUN Remote Procedure Call
ident	113/tcp	Identification Service
auth	113/tcp	Authentication Service
auth	113/udp	Authentication Service
nntp	119/tcp	Network News Transfer Protocol
nntp	119/udp	Network News Transfer Protocol
imap	143/tcp	Internet Message Access Protocol
imap	143/udp	Internet Message Access Protocol
snmp	161/tcp	SNMP
snmp	161/udp	SNMP
snmptrap	162/tcp	SNMPTRAP
snmptrap	162/udp	SNMPTRAP