Well, the HELO command is used on most machines to match your host to 
what you say in HELO, and marks the discrepency in the header, as well as 
the mail log.  You first establish a connection to a SMTP server, then 
you dump your email on it.  The mailing software will try to connect to 
the nearest SMTP machine to the final recipient, and a lot of times it 
will be the machine the recipient is on.

This list of socket connections is found in many computers as /etc/services
I got this one from ftp.whitehouse.gov.  Notice port 19 is chargen.  So say:
telnet whitehouse.gov 19      and you'll get a pretty pattern.

#
# @(#)services 1.16 90/01/03 SMI
#
# Network services, Internet style
# This file is never consulted when the NIS are running
#
tcpmux		1/tcp				# rfc-1078
echo		7/tcp
echo		7/udp
discard		9/tcp		sink null
discard		9/udp		sink null
systat		11/tcp		users
daytime		13/tcp
daytime		13/udp
netstat		15/tcp
chargen		19/tcp		ttytst source
chargen		19/udp		ttytst source
ftp-data	20/tcp
ftp		21/tcp
telnet		23/tcp
smtp		25/tcp		mail
time		37/tcp		timserver
time		37/udp		timserver
name		42/udp		nameserver
whois		43/tcp		nicname		# usually to sri-nic
domain		53/udp
domain		53/tcp
hostnames	101/tcp		hostname	# usually to sri-nic
sunrpc		111/udp
sunrpc		111/tcp
#
# Host specific functions
#
tftp		69/udp
rje		77/tcp
finger		79/tcp
link		87/tcp		ttylink
supdup		95/tcp
iso-tsap	102/tcp
x400		103/tcp				# ISO Mail
x400-snd	104/tcp
csnet-ns	105/tcp
pop-2		109/tcp				# Post Office
uucp-path	117/tcp
nntp            119/tcp         usenet		# Network News Transfer
ntp		123/tcp				# Network Time Protocol
ntp		123/udp				# Network Time Protocol
NeWS		144/tcp		news		# Window System
#
# UNIX specific services
#
# these are NOT officially assigned
#
exec		512/tcp
login		513/tcp
shell		514/tcp		cmd		# no passwords used
printer		515/tcp		spooler		# line printer spooler
courier		530/tcp		rpc		# experimental
uucp		540/tcp		uucpd		# uucp daemon
biff		512/udp		comsat
who		513/udp		whod
syslog		514/udp
talk		517/udp
route		520/udp		router routed
new-rwho	550/udp		new-who		# experimental
rmonitor	560/udp		rmonitord	# experimental
monitor		561/udp				# experimental
pcserver	600/tcp				# ECD Integrated PC board srvr
ingreslock      1524/tcp


ObHack:
School I work for (K-8) recently bought a laser disk player with a
computer interface.  The teachers were all excited about this
thing and wanted to use it with the computer &c.  So I told them the
the plain truth that they would have to cart the machine up and down the
stairs and hook it up to the various things it needs to.  They did not
like that idea at all.  So, my mentor and I banged heads all summer and 
remembered that one of our cable companies installed a video origination 
cable in the library.  We decided we'd use that video origination cable 
in the library and run a shared serial port over apple talk and run the 
whole laserdisk player as a server.

ObSetBack:

The librarian has to manually change disks.  The teachers don't like 
using the laser disk with the computer.  They like the barcode scanner 
instead.  So they bought one of those IR-to-radio remote control extender 
things.  Oh well.

