subnetting IP address

I don't know of any way for you to get that last subnet. The 255.255.255.128 netmask gives you two subnets (1-126 and 129-154). You cannot combine pieces of subnets, you have to take all or nothing.

If you do what you are describing, the machines with addresses 32-126 will not be able to access the machines 1-31 because they will think they are in the same subnet. You may be able to get around this with some fancy routing, but it's really not worth it.

If you need to create a network this complex, I would suggest that you use a private subnet for your network and do NAT translation at the router. This way, you could use the entire 172.16.0.0 class B for all of your machines and then assign the real Internet addresses to machines that need them (such as web servers). Any addresses that are left can be put into a pool for outgoing Internet access for the rest of your machines.

This sounds complicated, but it's really not. And it will give you more room for subnetting (each of your subnets can have a whole class C), as well as some additional security from the NAT setup.

You can't "overlap" or "extend" (depending on your point of view) subnets. But here's something that will work, assuming that you are using an IP routing protocol that supports VLSM (Variable Length Subnet Masks).

(Assuming your class C is 210.1.1.0)

210.1.1.128 with mask 255.255.255.128
- address range: 210.1.1.129 - 210.1.1.254
210.1.1.64 with mask 255.255.255.192
- address range: 210.1.1.65 - 210.1.1.127
210.1.1.32 with mask 255.255.255.224
- address range 210.1.1.33 - 210.1.1.63
You need to use all three subnet definitions to get the address range 210.1.1.33 - 210.1.1.254. You need to route between these three segments. Alternatively you can (on some routers) define secondary IP addresses. This is OK, but causes overlaps on the physical network which I believe results in excessive collisions, which will impact performance. better to stick with 3 distinct physical collision domains, and try to match a "workgroup" against the size of the subnet.

Finally you can takethe rest of your class C as follows:
210.1.1.0 255.255.255.252
- 210.1.1.1 -> .2
210.1.1.4 255...252
- 210.1.1.6 -> .6
210.1.1.8 255...252
- 210.1.1.9 -> .10
210.1.1.12 255...252
- 210.1.1.13 -> .14
210.1.1.16 255...252
- 210.1.1.17 -> .18
210.1.1.21 255...252
- 210.1.1.21 -> .22
210.1.1.24 255...252
- 210.1.1.25 -> .26
210.1.1.28 255...252
- 210.1.1.29 -> .30

(8 little 2-host subnets)

Hope that helps. For other info on Subnetting, try "learntosubnet.com".