You can also use symbolic prefixes to create special device names. For example, if your modem was on /dev/ser1, you could create a symbolic prefix of /dev/modem as follows:
ln -Ps /dev/ser1 /dev/modem
Any request to open /dev/modem will be replaced with /dev/ser1. This mapping would allow the modem to be changed to a different serial port simply by changing the symbolic prefix and without affecting any applications.