I'm reading up on subnetting in a cisco 'networking basics' book and I keep running across diagrams that show the IP address of a machine w/ something like FA0/0, S0/0/0 and I can't find what these mean or where they're explained in the book. I did find them in a table w/ a column header of 'output interface' but ... that really doesn't help me read the format.
Depends on the platform in use, and how they number interfaces on that platform. The simplest, smallest platforms like an old ancient 2501 or an 8xx series just have single #s. S0, S1, E0, A0. Bigger platforms that take modular cards tend to seperate out with slashs. So 0/0 is the card in slot0, the 1st interface on that card. Or, 0/0/0 could be the card in slot0, the WIC in spot 0 on the card in slot0, and the first interface on that WIC. If you get up to the BFR or 10k, you get one more layer deep, combine that with a channelized controller and a subinterface, and you can have things like 0/5/1/6:0.16.
You'll get a feel for it while just using a cisco router. Nothing magical about it. Ie. if you have a computer with a few hundred hard drives attached you'd get lost pretty quickly trying to figure out which particular hard drive is number 84. Lot easier to figure out which controller, shelf, and drive number in the shelf. Same sort of thing.
Its not really that important, they are just short abbreviations. Ethernet FastEthernet GigabitEthernet Serial HSSI ATM Loopback
Can't think of any other types of interfaces that regularly get abbreviated, although there's quite a few more.