I am managing what can only be described as the little network that grew. It began life with a linksys router, 3 windows machines and a dsl connection. Through the years, lots of servers, network printers, WAPS and the like have been added. if hiring continues at current pace, by the end of the year, we will have exhausted the complete 192.168.5.x with a single windows domain.
what kind of problems am I going to run into by moving workstations into another subnet for accessing internal servers, web etc. do i need to set up a different windows domain and then enable trust ? any help and advice from those who have done this before would be appreciated.
The easiest thing to do would be to reduce the Subnet Mask, so that the devices can access 192.168.6.x or 192.168.4.x without having to resort to the default gateway.
If you're running on DHCP (And if you've got 255 devices I sincerely hope you are) then changing the subnet mask should be simple enough, and then manually change the devices that are configured with static IPs.
That's not to say that if everything is running off cascading hubs and switches that you're not going to have other problems down the line (so perhaps this is an excuse to chuck in a proper router and have proper DMZ/private network segements)
This is mainly a networking problem, so you shouldn't need to do anything at the domain level, unless there are good reasons for doing that too; you can safely run a single Windows domain with thousands of users and computers, you just need to make good use of Active Directory OUs.
I agree with Faseeker: the best short-term solution would be to increase your subnet, moving to a larger one (255.255.254.0, 255.255.252.0 ... even 255.255.0.0 will do, you can use whatever you want in a private subnet). Just remember to modify the subnet mask on ALL your systems, otherwise some of them will not be able to talk to some other ones, causing a whole lot of very confusing problems.
For the long term, it would be wise to split the network into smaller subnets, like 192.168.1 for servers, 192.168.2 for printers, 192.168.3 for clients, etc.; but you will need at least one layer-3 switch for that.
If you have Internet-facing servers (web, mail, etc.) it could be good to consider using a DMZ for them.