So how is the client connected to the server right now? I'm a little confused by your description but to me it basically sounds like you want the client and the server to both be on the same lan and you want that lan to be connected to the web. Any router or layer 3 switch is what you are looking for. If you want to make the network wireless then buy a router with wireless built into it. What you have to consider is DNS and DHCP settings. If your server has DNS and DHCP roles enabled then you have to make sure the router/switch doesn't have it on or vice versa. If you want to give me some more details on what kind of server you're running I can help you out with that. I'm also assuming you have privileges to make changes to your works network. If you don't than absolutely under no circumstances should you make any changes to the configuration -contact your IT admin.
Modem -> Router/Switch -> Node/s (commonly, modems will already be built in with a router/switch and you can tell by looking at the back of the modem, there will be multiple rj-45 ports)(btw you shouldn't do this but it is possible to be connected through wifi and ethernet at the same time. each connectivity device has its own MAC address therefore if your dhcp is working properly each device will get it's own IP.)Edited by thx1138 - 12/9/11 at 8:49pm