I maintain a system at home in case I need to proxy my connection elsewhere or
otherwise need some Unix tools.  The system is behind a NetGear router which 
is directly connected to my cable modem.  The router is capable of using some
dynamic DNS services, but since my IP rarely changes, I just have a script 
setup on a system behind the router to check and send me a text message if my
public IP changes.  I run this script out of cron every 15 minutes.  It depends 
on lynx being installed (typical) and on systemj.net being reachable.


#!/bin/sh 
CACHE=/tmp/ip-check-cache.txt 
LASTIP=`cat ${CACHE}` 
CURRENTIP=`lynx --dump http://systemj.net/myip.xhtml | sed -e '/^$/d' -e 's/ //g'`
MAILTO="user@example.com"

if test "${LASTIP}" = "${CURRENTIP}"
  then exit 0
else
  echo "${CURRENTIP}" | tee ${CACHE} | mail ${MAILTO}
fi


When systemj.net was offline for a while I had a work-around setup with
traceroute; instead of the lynx command above I used this:

/usr/sbin/traceroute -n 72.29.100.156 2>/dev/null | head -1 | awk '{ print $2 }'

This was all done under NetBSD.