What to do when a command isn't found?

[Timothy Alberts]

So I get this every now and then and haven't found a smooth method of
dealing with it.  I have my desktop upgraded from FC4 to FC5 and the
command 'ifconfig' comes back with 'bash: ifconfig: command not found.'
Additionally, I have been through tutorials on the web and different
books on running commands that more often than not, the bash shell comes
back with the same message.

My first question is, are the shell commands and features standardized
somewhere so that I know what commands I can expect to have no matter
what machine I'm sitting at or what type of installation I have?  My
second question is, if these commands are supposed to be there, but the
shell comes back and says they aren't, what is the procedure for finding
the commands or installing the packages required for them?

I understand the 'which xxx' command will tell me the location of the
executable that is being used when I call a command.  However it doesn't
help me find a missing command.  I also understand that typically shell
commands are in /bin /usr/bin /usr/local/bin among others.  If it helps,
when I type 'which ifconfig' the following is the result:

/usr/bin/which: no ifconfig in
(/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/bin:/usr/bin:/bin:
/usr/X11R6/bin:/home/talberts/bin)

Sorry these are fairly general questions so if it's easier to just focus
on the 'ifconfig' command that would be helpful.  Thank you for any
response.