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After you've been dealing with Internet services for a while, you come to remember certain well-known port numbers off of the top of your head. SMTP (Section 46.8) is port 25, HTTP is port 80, and so on. However, unless your memory is far better than mine, you won't remember them all.
Luckily, that's part of what /etc/services is for. It's a database of well-known ports with symbolic names; any program that can take a port number should be able to take the appropriate symbolic name instead. If you want to make sure your SMTP server is up, the following two commands are equivalent:
% telnet localhost 25 % telnet localhost smtp
The definitive database of well-known ports is currently available at http://www.iana.org/assignments/port-numbers. On most Unixes, /etc/services is just a snapshot taken at the time that version of that Unix was released. When installing new services, often you'll want to tweak your local copy of /etc/services to reflect the new service, if it's not already there, even if only as a reminder.
The format of the /etc/services is simple:
service name port/protocol aliases
Comments within the file start with a pound sign (#). As an example, a few common entries from /etc/services:
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 #Secure Shell Login ssh 22/udp #Secure Shell Login telnet 23/tcp telnet 23/udp smtp 25/tcp mail #Simple Mail Transfer smtp 25/udp mail #Simple Mail Transfer
-- DJPH
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| 46. Connectivity |  | 46.3. Status and Troubleshooting |
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