How many times have we all seen an error message like modprobe: can't locate module net-pf-5 at boot time, and wondered what it meant? I used to get this all the time when I chose to leave Appletalk support out of my kernel. Something (in my Red Hat system, at least) tried to access it anyway, and kerneld (or kmod these days) tried to load the corresponding module.
It's easy enough just to make these errors go away, by putting a line like alias net-pf-5 off into /etc/conf.modules. But what if it was something important? How do I know that net-pf-5 corresponds to something as useless (to me) as Appletalk? Fortunately, David Yaffe found a handy list of these numbers hiding in the kernel sources, in /usr/src/linux/include/linux/socket.h. For those of you who don't feel like digging around in there, here it is, in a friendlier format. The comments are taken verbatim from the source.
|net-pf-1||UNIX||Unix domain sockets|
|net-pf-2||INET||Internet IP Protocol|
|net-pf-3||AX25||Amateur Radio AX.25|
|net-pf-6||NETROM||Amateur Radio NET/ROM|
|net-pf-9||X25||Reserved for X.25 project|
|net-pf-10||INET6||IP version 6|
|net-pf-11||ROSE||Amateur Radio X.25 PLP|
|net-pf-12||DECnet||Reserved for DECnet project|
|net-pf-13||NETBEUI||Reserved for 802.2LLC project|
|net-pf-14||SECURITY||Security callback pseudo AF|
|net-pf-15||KEY||PF_KEY key management API|
|net-pf-22||SNA||Linux SNA Project (nutters!)|