Firewall question

[Anne Wilson]

In another thread Tim Evans said:

You can, however, configure iptables to *allow* only a specified list of
IP addresses (i.e., the ones you approve of).

This set me wondering.  If you use wifi hotspots to collect mail, for 
instance, you have no idea what IPs you will be using.  Is there any way that 
the system can recognise the computer rather than the IP?  I'm guessing it 
must be possible, as my on-line bank knows immediately if I use a computer 
that I haven't used before.

Anne