From what I can tell, there's not enough description of the problem to diagnose and suggest a solution.

You appear to be talking about two completely different things.

First, there's the subnet used for the TCP/IP configuration as handed out by the DHCP server.

For a start, if that's wrong the problem lies with the DHCP server, not with the Mac. The network administrator needs to make sure the DHCP server is handing out the right subnet.

That said, you can manually change it anyway, regardless of what the DHCP Server provides. Just open a terminal window and run (assuming built-in ethernet):

sudo ifconfig en0 netmask <netmask>

where <netmask> is the netmask you're supposed to use.

Now, the second problem you describe is AppleTalk since they're running switches/routers that are not handling AppleTalk.

No biggie. All recent versions of AppleTalk have supported the ability to run over a TCP/IP connection. For example, if you want to connect to an AppleShare server, you can 'Connect to Server... afp://ip.address.dom/' and it'll run over a TCP/IP connection.

What you do lose with this setup is the ability to browse the network and see available servers, printers, etc. that aren't on the local AppleTalk subnet, but that's about it.

Neither of these issues should be showstoppers.