There are a number of places where you can find good information about Linux networking.
There are a wealth of Consultants available. A listing can be found at LinuxPorts Consultants Database
Alan Cox, the current maintainer of the Linux kernel networking code maintains a world wide web page that contains highlights of current and new developments in linux Networking at: www.uk.linux.org.
Another good place is a book written by Olaf Kirch entitled the Network Administrators Guide. It is a work of the Linux Documentation Project and you can read it interactively at Network Administrators Guide HTML version or you can obtain it in various formats by ftp from the metalab.unc.edu LDP ftp archive. Olaf's book is quite comprehensive and provides a good high level overview of network configuration under linux.
There is a newsgroup in the Linux news hierarchy dedicated to networking and related matters, it is:
There is a mailing list to which you can subscribe where you may ask questions relating to Linux networking. To subscribe you should send a mail message:
To: firstname.lastname@example.org Subject: anything at all Message: subscribe linux-net
On the various IRC networks there are often #linux channels on which people will be able to answer questions on linux networking.
Please remember when reporting any problem to include as much relevant detail about the problem as you can. Specifically you should specify the versions of software that you are using, especially the kernel version, the version of tools such as pppd or dip and the exact nature of the problem you are experiencing. This means taking note of the exact syntax of any error messages you receive and of any commands that you are issuing.