[lug] Tape backup error

[Bob Collins]

I want to backup across my local area network.  I am testing by 
backing up to a file instead of tape.  

I can telnet or ftp to linux.local.  If I try:

tar -cvf linux.local:/home/bob/bkup.tar /etc

I get:

tar: linux.local:/home/bob/bkup.tar: Cannot open: Input/output error
tar: Error is not recoverable: exiting now

What port does tar use.  I thought it might be the ftp or the telnet 
port.  I can find only that the default port is used.  Where can I find
out about this?  Or am I completely in the dark? 

As always all help appreciated.

Bob