reverse grep

[deeptech71 at gmail.com]

How is it possible to select lines that do NOT match a specific pattern?

For example, I'm connecting to 192.168.123.254 via telnet (port 23), and 
do tcpdump -nli rl0. This cyclic traffic, becuase when tcpdump outputs 
something, the system sends me some packets, which generates output in 
tcpdump, and vice versa. I want to filter out packets of telnet access 
to the FreeBSD machine, that is, something like:

     tcpdump -nli rl0 | grep --non-matching-lines 192.168.123.254.23