Why do my local network ip addresses keep changing?

[joe at actionline.com]

Thanks to everyone who responded to this question.  I'm still trying to
digest all the information.

Meanwhile, I discovered that by changing the destination ip address in my
main computer to the one that ifconfig showed it to be in the remote
computer to which I was trying to scp some files, that did not work.  So,
after much fooling around, I ended up rebooting the destination computer
and lo and behold the ip address became restored to what it had previously
been and everything worked.

It all remains a great mystery to me ... but everything now works again.