Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 88
CIDR and VLSM Comparison

CIDR and VLSM seem similar; in essence, they are. Why not use VLSM instead of CIDR? The difference is that CIDR allows for the efficient routing mechanism to take place by the recursive allocation of an address block. Routing is then based on this address block allocation and not on an individual Class address. This block is handed down by the IANA to the IR, to the upper-level ISP down through the ranks of downstream ISPs, and, finally, to the customer.

VLSM permits recursion as well, but more so on an individual address space in use by the customer. A customer division of an address space is not visible by the Internet. VLSM still operates with Class addresses.

Variable-length masks allow for variable-length subnets per network ID based on an address assignment by an ISP. This allows one network number to contain different masks and is a better use of an IP address. With VLSM, a lot of the bits in an address space are wasted. The example is assigning an IP address to a point-to-point WAN link, which wastes 252 address bits.

This allows for greater flexibility when dividing up a network ID into subnets and hosts. Without VLSM, you have to choose between having enough networks, with close to the right amount of hosts, or having the right amount of hosts with close to the right amount of networks.