VLSM and CIDR

CIDR (Classless Inter-Domain Routing) was introduced in 1993 replacing the previous generation of IP address syntax, classful networks. CIDR allowed for more efficient use of IPv4 address space and prefix aggregation, known as route summarization or supernetting. 

With CIDR, address classes (class A, class B, class C) became meaningless. The network address was no longer determined by the value of the first octet, but assigned prefix length (subnet mask). Address space, the number of hosts on a network, could now be assigned a specific prefix depending upon the number of hosts needed for that network.

CIDR allows for supernetting. A supernet is a group of major network addresses summarized as a single network address with a mask less than that of the default classful mask.

CIDR uses VLSM (Variable Length Subnet Masks) to allocate IP addresses to subnetworks according to need rather than by class. VLSM allows for subnets to be further divided or subnetted into even smaller subnets. Simply put, VLSM is just subnetting a subnet.

Propagating CIDR supernets or VLSM subnets require a classless routing protocol. A classless routing protocol includes the subnet mask along with the network address in the routing update. 

Determining the summary route and subnet mask for a group of networks can be done in three easy steps. The first step is to list the networks in binary format. The second step is to count the number of left-most matching bits. This will give you the prefix length or subnet mask for the summarized route. The third step is to copy the matching bits and then add zero bits to the rest of the address to determine the summarized network address. The summarized network address and subnet mask can now be used as the summary route for this group of networks. Summary routes can be used by both static routes and classless routing protocols. Classful routing protocols can only summarize routes to the default classful mask.