wow
all of you are completely wrong in this thread, sorry not trying to be a dick, but let me set a few facts for you all.
1) CSS does not need to run on a sever OS, in fact it is probably not recommended. Also a server OS cost a lot for like a 5 user license, its nearly $1,000 dollars, so that would be retarded to even consider it.
2) You do need to be on the same ip range and subnet to be networked over TCP/IP, this is standard
3) Anyone who suggested buying windows server for hosting a LAN is, well misinformed.
The best way to get everyone internet access, and the best way to get everyone on the same IP scheme and subnet is to just hook up that 24 port switch into a router. A router will act as a DHCP server giving every client their own IP on the same subnet as everyone else. You can even set the range to like 192.168.1.50 ~ 100 allowing 50 clients to connect to your network. Then you can set up what is called static DHCP on the machine that is your server, and give it an ip of lets say for arguments sake, 192.168.1.10 (192.168.1.1 would most likely be your router) then anyone who wanted to map any network shares to your server or what not could by the IP. Then you can download and install HLDS on the machine that is the dedicated server which should either be running a client version of windows (ie windows xp) or Linux since HLDS can be ran on both platforms.
All of this can be easily accomplished and configured by using a $50 dollar router, and better yet if you get one compatible with some third party firmware like DD-WRT you can do a lot cooler things down the road. So, if you got out and spend $1,000 on a 5 user license for windows server not only would I laugh at you for wasting all that money, you should probably be spending your money on better things.
Server OSes are not meant for running games, plus any machine can act as a dedicated server. I think a lot of you are not fully getting what each of these means. they are the same words but really mean two different things. A server OS is meant for management, and you find those in businesses and Educational organizations, etc. A dedicated server is just any old client machine that is sacrificed, ie not playable, for your video game. Not to be confused with what a server does in an enterprise level network.