Network interfaces

eth0 is the interface that shows up when you plug an USB to ethernet adapter. A list of compatible ones is located here.
wlan0 is the embedded Wi-fi interface.
wlan1 is the name of a Wi-fi dongle.
usb0 is the big USB port (not sure, may be the first plugged).
usb1 is the small USB port (not sure).

Anyway, you can check the physical link connectivity, as root: ethtool usb0 ethtool usb1 ethtool eth0

If you connect an OP with another using an USB cable, the OP at the little end of the cable will dramatically drain the current from the other OP's big USB port.
To avoid this, at the little end, type as root:

echo 0 > /sys/class/power_supply/twl4030_usb/enable

there's also a bluetooth interface, but until now i never used it, so can't tell much about it.
These interface numbers may vary, depending which one you plug first.
The numbers can be fixed using the /etc/udev/rules.d/70-persistent-net.rules file.
This file is deleted at each shutdown. To make use of it, edit /etc/rc.d/rc.local_shutdown , and comment this line:

rm /etc/udev/rules.d/70-persistent-net.rules

into

#rm /etc/udev/rules.d/70-persistent-net.rules

SL4P default setup

The Network Manager, well, manages the connections, and asks for an IP address through a DHCP request. This is how it acts when you boot SL4P for the first time.
You can set it up through a right-click on the Xfce desktop icon.
It's not included in the stock Slackware.

Wicd is included in the stock Slackware extra packages, and i shipped it in SL4P.
I tried it only a bit so i can't recommend it, or not.
If you want to give it a try, be root, then disable the network manager. Edit /etc/rc.d/rc.local and comment at the end:

/etc/rc.d/rc.networkmanager start

into

#/etc/rc.d/rc.networkmanager start

chmod +x /etc/rc.d/rc.wicd

Activate Wicd frontend under Xfce. Go in the menu -> Settings -> Session and Startup -> Application Autostart tab, then check "Wicd Network Manager Tray".
Reboot.

Stock Slackware doesn't use the Network Manager, nor Wicd. The network is established at boot, once for all.
If you want a non-graphical server-oriented setup, you have to disable the network manager (and Wicd). Edit /etc/rc.d/rc.local and comment at the end:

/etc/rc.d/rc.networkmanager start

into

#/etc/rc.d/rc.networkmanager start

netconfig

darkstar

otherwise you'll break some Pandora scripts compatibility. Sudo won't launch anything with the wrong hostname.

Set you parameters.
Edit /etc/rc.d/rc.inet1.conf to finalize your setup.
Be sure to have all used interfaces plugged in when booting. With the deactivation of the network manager, it's no more plug and play.
Reboot.