Yet another implementation of TCP/IP networking comes from Matthias Urlichs, who wrote an ISDN driver for Linux and FreeBSD. For this, he integrated some of the BSD networking code in the kernel.
For the foreseeable future, however, Net-3 seems to be here to stay. Alan currently works on an implementation of the AX.25 protocol used by ham radio amateurs. Doubtlessly, the yet to be developed ``module'' code for the kernel will also bring new impulses to the networking code. Modules allow you to add drivers to the kernel at run time.
Although these different network implementations all strive to provide the same service, there are major differences between them at the kernel and device level. Therefore, you will not be able to configure a system running a Net-2e kernel with utilities from Net-2d or Net-3, and vice versa. This only applies to commands that deal with kernel internals rather closely; applications and common networking commands such as rlogin or telnet run on either of them.
Nevertheless, all these different network version should not worry you. Unless you are participating in active development, you will not have to worry about which version of the TCP/IP code you run. The official kernel releases will always be accompanied by a set of networking tools that are compatible with the networking code present in the kernel.