Secondly, firewalls do not protect against back doors into the site. For example, if unrestricted modem access is still permitted into a site protected by a firewall, attackers could effectively jump around the firewall [Haf91]. Modem speeds are now fast enough to make running SLIP (Serial Line IP) and PPP (Point-to-Point Protocol) practical; a SLIP or PPP connection inside a protected subnet is in essence another network connection and a potential backdoor. Why have a firewall if unrestricted modem access is permitted?