Sample Paper on Classless Inter Domain Routing and Variable Length Subnet Mask



Classless Inter Domain Routing (CIDR) and Variable Length Subnet Mask (VLSM) are concepts used in networking and while used interchangeably, they are both similar and different. CIDR and VLSM both allow the division of the IP address space into small pieces hence allowing for the efficient use of IP space. They also allow for greater flexibility in creating sub networks by overcoming the restrictions of the A-B-C class mechanism (Chen, 2008). They are also useful in the scalability of the network space to various client requirements in an economical way. Both mechanisms are also used with classless routing protocols and require three prerequisites for their successful implementation:

  • The addresses must be assigned in a topologically significant way in order for route aggregation to occur
  • A consistent forwarding algorithm that is based on the longest match must be implemented by all routers
  • During each route advertisement, the routing protocols must contain network prefix information.

CIDR and VLSM also have differences, the biggest being that VLSM deals with subnetting (breaking up networks into smaller parts) while CIDR combines networks together by adjusting the subnet mask. Additionally, CIDR is applied at the ISP/Provider level to allocate an address block to a higher level, mid-level, and lower-level ISPs. VLSM, on the other hand, is used by the recipients of an already assigned address block to utilize it more efficiently and is invisible to the global internet. Moreover, while CIDR can be used in converting classful networks, VLSM requires the address blocks to be classless first before it can be implemented. CIDR and VLSM thus have the same task of optimizing networks but where CIDR combines them, VLSM divides them.

Why they are needed on the same network

CIDR and VLSM were introduced in order to address the challenges of the A-B-C Class mechanism that saw the filling up of IP address space. These challenges included the exhaustion of the Class B network address space, the eventual exhaustion of the 32-bit IPv4 address space, and the rapid growth in the size of the internet’s routing tables. The IPv4 address space was running out because a lot of addresses were unassigned and thus wasted, leading to the introduction of CIDR and VLSM into all modern networks. In implementing the two, CIDR is first used by the ISPs to support route aggregation. CIDR is basically supernetting, where thousands of traditional classful routes are grouped together and represented by a single routing table and subnet mask. A single routing table entry thus specifies how traffic will be routed to many individual network addresses (Bornhager, 2002). The ISP can thus carve out address space that specifically meets the needs of each client hence educing wastage. CIDR is also used to reduce route flapping and control the amount of data in the backbone routers.

At the recipient level, a network administrator uses VLSM to break the subnet or supernet into smaller parts to be used by individual households. VLSM thus allows the optimization of the assigned address block by breaking up the network into unequal pieces that can be used more efficiently. While CIDR improves efficiency by allowing one routing table, VLSM further improves on this efficiency by reducing the number of entries in the routing table. CIDR and VLSM have been used together in networks to improve the efficiency and scalability of networks and have reduced the administrative burden of maintaining networks


Bornhager, M. (2002). VLSM and CIDR. Cisco Systems (pp. 10-36). Halmstad University.

Chen, T. (2008). VLSM and CIDR- Routing Concepts and Protocols. California: Cisco Networking Academy.