Couple of questions

Hello world!

I wonder if someone could explain to me in simple English what subnet mask
is and how to apply it to IP address.
Second thing is...Is there an advantage of using L2TP (no IPSec) vs. PPTP?
Third...what is ATM...like in PPoA (ADSL connections).And those vpi/vci
settings...

Thanks in advance,

--
Yours truly,
Alon Brodski