The mode command is the last command executed by dip before gong into daemon mode. Unless an error occurs, the command does not return.
mode takes a protocol name as argument. dip currently recognizes SLIP and CSLIP as valid names. The current version of dip does not understand adaptive SLIP, however.
After enabling SLIP mode on the serial line, dip executes ifconfig to configure the interface as a point-to-point link, and invokes route to set the route to the remote host.
If, in addition, the script executes the default command before mode, dip will also make the default route point to the SLIP link.