Classless InterDomain Routing (CIDR)

Zaman,
These can be easily confused–in fact I had to review them myself to make sure I am giving you accurate information.

VLSM is aptly named, because it means using a subnet mask of variable lengths throughout your organization with the same network block. For example, suppose your ISP has given you 10.10.10.0/24 as an IP range. Without VLSM, you would have to pick a certain subnet mask, say, 255.255.255.240 (/28) and stick with that. So if you wanted to use this block throughout your company, you would always have to use /28. This becomes wasteful in the case where you have a subnet with just two hosts, or it becomes inadequate where you have a subnet with 50 hosts. With VSLM, you have the freedom to change your subnet mask as needed, so in the case of a subnet with two hosts, you could use 10.10.10.248/30, for example, and with a subnet having 50 hosts, you could use 10.10.10.0/26, for example.

CIDR is basically “supernetting.” This is where you can aggregate networks together into larger networks beyond their natural network boundary. For example, suppose I have 192.168.0.0/24 and 192.168.1.0/24. These networks have a natural /24 boundary (because they are class C addresses), but I can using CIDR aggregate them via 192.168.0.0/23