Here's a method for logging on to SAPNet directly through SAPGUI, without logging in to your R/3 system and using transaction OSS1. The benefit of this is that you can access SAPNet even if your R/3 system isn't available for some reason.
(This assumes you can connect to SAPNet via OSS1)
1. To determine PC call parameters, run transaction OSS1
2. Menu path Parameter>Technical Settings, then menu path Parameter>PC Call Parameters (call parameter string is displayed)
3. Copy the PC SAP GUI call parameters to clipboard
4. Create an icon on your PC that calls SAPGUI with these parameters and edit the target so that it launches SAPGUI with your PC call parameters (e.g. "c:progra~1SAPpcSapGuiSAPGUI.EXE /H/10.0.0.1/S/sapdp99/H/126.96.36.199/S/sapdp99/H/oss002/S/sapdp01")
5. You can now log on to SAPNet directly via the SAPGUI icon on your desktop!