the kernel doesn't just shove IPpackets through it. There is a
protocol called `Point-to-Point Protocol', or `PPP', which is used to
negotiatewith the other end before any packets are allowed through.
This is used by the ISP to identify who is dialed up: on your Linux
box, a program called the `PPP daemon' handles your end of the
negotiation.
Because there are so many dialup users in the world, they usually
don't have their own IP address: most ISPs will assign you one of
theirs temporarily when you dial up (the PPP daemon will negotiate
this). This is often called a `dynamic IP address', as separate from a
`static IP address' which is the normal case where you have your own
address permanently. Usually they are assigned by modem: the next time
you dial up,you will probably get a dierent modem in the modem pool,
and hence a dierent IP address.
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