The mask associated to a PIX interface, or any other network equipements or hosts even Win hosts (except dialup), determined the subnet range. To be able to communicate, you need to have at least 2 hosts on the same subnet or the same range, otherwise the layer 2 protocol won't be able to reach the other host on this subnet.
The dialup case, is particular, because Microsoft doesn't respect the principle of subnet within DUN (Win's Dialup Network driver). Microsoft, even, doesn't care of the gateway's IP address provided by the access server, instead MS put the IP address of the host in the gateway field. That is, a Win host always send the packets to the other side of the PPP network (dialup connection) when they are destined to someone else than himself. It's working when the dialing host has only 1 interface but what's happen when this host needs to communicates with a dialup network through a modem and with a LAN simultaneously. In MS world, this doesn't work. You need special kind of software to bypass this bad implemention. Just to say that the net mask hasn't any significance in dialup MS world, you can find 255.255.255.255.
Regards,
Ben
