VLSM vs CIDR

Trenton Moody

Ok I understand in depth what CIDR is and how to create different networks. I have a vague Idea what VLSM is. Can someone pretty please explain to me the difference or similarities.

todd-murphy

VLSM (Variable Length Subnet Masking) is basically just using different length subnet masks a single network.

For example, you have two networks and only a limited range of addresses allocated to you.
Network A ) 500 Hosts
Network B ) 16 Hosts

Network A has too many hosts for a typical /24 (255.255.255.0) so you instead provide it with a /23 (255.255.254.0).
Network B on the other hand doesn't have many hosts at all, so in the interest of conserving IP Addresses you might assign it a /27 (255.255.255.224).

This is basically all VSLM is, combining subnets of different sizes on the same network typically for reason's such as conserving IP's, accommodating large networks, minimising broadcast traffic, security reasons or so on.

Ronnie Wong

@Trenton-Moody,

VLSM is a methodology;CIDR is a way of reporting.

We use VLSM to make more efficient subnets. Taking large subnets and subdividing them to smaller networks. Creating a more efficient network addressing space than the classful networking. We do so changing the number of bits from the default classful subnet masks to whatever we want them to be, i.e. We break the rules of classful networking boundaries. Normally to show this we would see subnet masks like 255.240.0.0 or 255.255.255.192.

VLSM, along with NAT (Network Address Translations) and RFC 1918 (Private IP Addressing) are 3 of the technologies that are used to slow the depletion of the IPv4 Address Space on the Internet. Though this is primary use for VLSM, e also found its usefulness in managing IP Address space.

CIDR is a way of reporting the VLSM that make's it easier for us to use make logical reference to the smaller networks too. Usually in a production network, we see CIDR reported when we know that we be performing some network summarization or if we've already performed so network summarization. We normally see /12 or /27 instead of reporting the subnet mask. This "slash notation" represents the number of bits used for the subnet mask instead of reporting the subnet mask as in the previous paragraphs.