Difference Between VLSM and CIDR

VLSM – Variable Length Subnet Masking. Several new methods of addressing were created so that usage of IP space was more efficient. The first of these methods is called Variable-Length Subnet Masking (VLSM). Subnetting had long been a way to better utilize address space. Subnets divide a single network into smaller pieces. This is done by taking bits from the host portion of the address to use in the creation of a “sub” network. For example, take the class B network The default network mask is, and the last two octets contain the host portion of the address. To use this address space more efficiently, we could take all eight bits of the third octet for the subnet.

One drawback of subnetting is that once the subnet mask has been chosen, the number of hosts on each subnet is fixed. This makes it hard for network administrators to assign IP space based on the actual number of hosts needed. For example, assume that a company has been assigned and has decided to subnet this by using eight bits from the host portion of the address. Assume that the address allocation policy is to assign one subnet per department in an organization. This means that 254 addresses are assigned to each department. Now, if one department only has 20 servers, then 234 addresses are wasted.

Using variable-length subnet masks (VLSM) improves on subnet masking. VLSM is similar to traditional fixed-length subnet masking in that it also allows a network to be subdivided into smaller pieces. The major difference between the two is that VLSM allows different subnets to have subnet masks of different lengths. For the example above, a department with 20 servers can be allocated a subnet mask of 27 bits. This allows the subnet to have up to 30 usable hosts on it.

CIDR – Classless Inter-Domain Routing. CIDR is also called supernetting. It’s an IP addressing scheme that replaces the older system based on classes A, B, and C. With CIDR, a single IP address can be used to designate many unique IP addresses. A CIDR IP address looks like a normal IP address except that it ends with a slash followed by a number, called the IP prefix. For example:

The IP prefix specifies how many addresses are covered by the CIDR address, with lower numbers covering more addresses. An IP prefix of /12, for example, can be used to address 1,048,576 former Class C addresses.

CIDR addresses reduce the size of routing tables and make more IP addresses available within organizations.

Comparing CIDR to VLSM
CIDR and VLSM both allow a portion of the IP address space to be recursively divided into subsequently smaller pieces. The difference is that with VLSM, the recursion is performed on the address space previously assigned to an organization and is invisible to the global Internet. CIDR, on the other hand, permits the recursive allocation of an address block by an Internet Registry to a high-level ISP, a mid-level ISP, a low-level ISP, and a private organization’s network.

Difference between Classful and Classless Routing


* Classful routing protocols strictly follow the subnet masks i-e. for Class A (8-bit prefix or /8), B (16-bit prefix or /16), and C (24-bit prefix or /24).
* Do not carry subnet mask information on their routing updates. This makes them unsuitable for hierarchical addressing that require Variable Length Subnet Mask (VLSM) and discontiguous network.
* All devices in the network must use the same subnet mask therefore when running a classful routing protocol on a network, make sure you use the same subnet mask everywhere. Otherwise, routing black holes can occur.
* RIP v1 and IGRP are classful routing protocols.


* Classless routing protocols do carry subnet mask information on their routing updates.
* Allow summarization of routes into smaller, more manageable groups.
* Classless routing is also known as supernetting or Classless Inter-Domain Routing (CIDR).
* Classless routing protocols extend the standard Class A, B, or C IP addressing scheme by using a subnet mask or mask length to indicate how routers must interpret an IP network ID.
* Classless routing protocols include the subnet mask along with the IP address when advertising routing information. Subnet masks representing the network ID are not restricted to those defined by the address classes, but can contain a variable number of high-order bits. Such subnet mask flexibility enables you to group several networks as a single entry in a routing table, significantly reducing routing overhead.
* Classless routing protocols includes RIP v2 and OSPF, Border Gateway Protocol version 4 (BGP4) and Intermediate System to Intermediate System (IS-IS).

Executing "show" Commands in Global Configuration Mode – EXEC Commands in Configuration Mode

In Cisco IOS, EXEC mode commands cannot be run from the Global Configuration mode or in any other configuration mode. This means, everytime an admin makes a change from the configuration mode (Global Config) then you have to “end” or “Ctrl+Z” back to the EXEC mode and run the EXEC commands like “show”, “clear” and “debug” commands.With the “do” command thats a thing of the past. The “do” command can be used to run any EXEC commands from within Global Configuration mode or any other Configuration mode without having to end the config mode.

For example:
Router(config)#do show running-config