You cannot duplicate addresses that may already be in use. For example:
If you are given a network address from your ISP, lets say 190.168.10.32/27. The ISP says that you can ONLY use this network address range. All other numbers are in use. You can stick with a flat network of 30 hosts, OR re-subnet the number into smaller networks with fewer hosts, which is VLSM. You can change the mask to /28, /29, or /30 if you want, although /30 are usually saved for P2P router connections.
So, lets take 190.168.10.32/27 and make it 190.168.10.32/29. That gives us four networks of 6 hosts per network rather than the flat 30 host network foisted on us by the ISP:
190.168.10.32 First host .33 last host .38, broadcast .39
190.168.10.40 First host .41 last host .46, broadcast .47
190.168.10.48 First host .49 last host .54 broadcast .55
190.168.10.56 First host .57 last host .62 broadcast .63
Whilst you are correct that the /29 does indeed stop at 248, we have to stop where we have because the ISP has told us that all the other possible network addresses and combinations are in use by others. So we have taken our original flat 30 host network and resubnetted it into 4 networks with 6 hosts per network, and we haven't nicked any numbers that are in use by others as per the ISP's conditions.
All we need now are some VLAN's and some DOT1Q trunking and maybe some subinterfaces, and we're off to the small business network design awards with a strong possibility of winning something!
