For a really good networking time, try "ethereal", a superb GUI 
network packet/stream analyzer.  It understands the protocols 
of lots of different kinds of packets, and displays their 
contents for you in human-readable form.  It will collect the 
content of all the packets of a TCP/IP stream and show it to 
you in ASCII.  Extremely educational!  Ideal for figuring out, 
e.g., exactly why your mail client is not communicating with 
your ISP's POP or SMTP server, and is only giving you strange
error messages.  Ethereal is newbie-friendly!

  http://www.ethereal.com

Here's a command line for starting it with some good options
(and, if you're on a cable Internet account that floods your NIC
with ARP packets, obscuring everything else, filters those out).
If you're on a dialup modem account, change eth0 to ppp0.  You
have to be root.

  ethereal -i eth0 -k -S -t a -f 'not arp'

If ethereal is your network eyes, netcat is its complement, 
your network hands.  Netcat connects to a remote host and port 
that you specify, and whatever you shove into its stdin gets 
sent out to there, and whatever comes back comes out of its 
stdout.  In other words, it allows you to do any network 
operation you want from a shell script!  I think you can make 
it interactive by using named pipes.  

  http://www.atstake.com/research/tools/network_utilities/

Typical procedure:

You have a canned client, which accesses some kind of data from 
a network server and does something with it beyond your control.  
You want to get that data and do what you will with it.

1. Run ethereal, run the client, and examine and understand the
traffic between them.

2. Write a script using netcat that (using the data from step 1)
pretends to be the client, but captures the data and hands it 
to you.

Networking documents -- I learned what I know of networking from 
these excellent (ASCII) docs and from examining smtp, pop3, http, 
and mp3-stream operations with ethereal.  They will take you from 
zero knowledge to comfortable familiarity.

  ftp://athos.rutgers.edu/runet/tcp-ip-intro.doc
  ftp://athos.rutgers.edu/runet/tcp-ip-admin.doc

begin  <m%3ub.25614$Dw6.129365@attbi_s02>,
	mark@cosmicpenguin.com (Mark S Bilk) writes:
>
> For a really good networking time, try "ethereal", a superb GUI
> network packet/stream analyzer.  It understands the protocols of
> lots of different kinds of packets, and displays their contents for
> you in human-readable form.  It will collect the content of all the
> packets of a TCP/IP stream and show it to you in ASCII.  Extremely
> educational!  Ideal for figuring out, e.g., exactly why your mail
> client is not communicating with your ISP's POP or SMTP server, and
> is only giving you strange error messages.  Ethereal is
> newbie-friendly!
> 
>   http://www.ethereal.com

Ethereal is indeed an excellent tool. Another great feature is that it
will read snoops made using tcpdump. We have a lot of Nokia firewalls
and I'm often running tcpdump on 2 or more interfaces looking for
network problems. Copying the tcpdump files to my laptop and analysing
them with ethereal is brilliant.

As with any SW ensure you have the latest version installed. There
have been 2 or 3 security bugs found in ethereal over the past year or
so.