You are correct. With a mask of 255.255.0.0, the 10.20.30.x and 10.20.31.x machines will be able to talk to each other directly, because they are both part of the same 10.20.x.x subnet.
With a mask of 255.255.255.0, they would be on different subnets, and would require the services of a router or Layer 3 switch.
If you add other ranges of numbers in the future such as 10.20.32.x etc., but keep the same 255.255.0.0 mask, you will not need a router or Layer 3 switch.
If you change the mask, then depending on the size of the mask you may or may not need routing services between them. For example, a mask of 255.255.128.0 would still allow the .30.x, .31.x, and .32.x machines to talk to each other; but they would have to go through a router to talk to anybody with addresses .128.x or .129.x or higher.
A mask of 255.255.192.0 would still let your .30.x, .31.x, and .32.x talk to each other, but they would need to use a router for anything .64.x and higher.
A mask of 255.255.224.0 would let .30.x and .31.x talk to each other, but they would need a router to talk to .32.x and anyone higher than that.
A mask of 255.255.240.0 would let .30.x and .31.x talk to each other directly, but they would need a router to reach anyone .32.x and higher, and also anyone .15.x and lower.
In fact, .30.x and .31.x will be able to talk directly to each other, up to the point where the mask is 255.255.254.0. But they will need a router more and more to reach machines that are not in their subnet, because the maximum size of that subnet is shrinking the more you extend the mask. And as stated earlier, with mask 255.255.255.0 you need the router for communications between any of those three subnets, as well as communications from those subnets to other subnets not mentioned here.
If you have switches with Layer 3 capability, you can set them up to route internally between VLANs. On a 4003 chassis, for example, you can do this with the 4232 L3 switch module. I haven't looked at the 4500 chassis to see whether you do Layer 3 on the supervisor module or on a L3-capable line module like the 4232.
Recommendation:
Cisco Top-Down Network Design book recommends that no more than 200 machines be active on a broadcast domain (VLAN) that's heavily using IP protocols, or if multiple desktop protocols such as IPX or AppleTalk are involved. This matches nicely with the 255.255.255.0 IP subnet mask, which permits up to 254 usable addresses per subnet.
Plan on designing your network so that you mask your machines this way from the beginning, or else you will have to go around to each one and change them later. Use VLANs to separate things on your network in some logical fashion, for example by wiring closet, department, etc.
Hope this helps.
