Why don't SLIP and PPP require a complete physical addressing system such as the system used with ethernet?
A point-to-point connection doesn't require an elaborate physical addressing system such as ethernet's because only the two computers participating in the connection are attached to the line. However, SLIP and PPP do provide full support for logical addressing using IP or other Network layer protocols.
Why does PPP use NCP protocols to configure protocol-specific settings instead of configuring protocol-specific settings during the LCP connection establishment phase?
LCP undertakes only configuration tasks that are common to all protocols. Individual protocols are then configured through NCP packets. This modular arrangement minimizes startup time because only the protocol settings that are necessary will be configured.
Why wouldn't SLIP be a good choice for a typical dial-up Internet service connection?
Most ISPs assign temporary IP addresses at connect time. SLIP doesn't support dynamic IP address assignment, so it wouldn't be a good choice for a typical dial-up account.