CCNA level doubt regarding VLSM

Answered Question
Feb 21st, 2008

I read about VLSM and understood its importance is conserving IP space in an enterprise.

What i am wondering is if an enterprise is assigned a class A private IP range(practically more than enough IP addresses that the enterprise can imagine to use), will it still be useful to use VLSM?

What i understand is using VLSM here will only increase the routing table size and cause added complexity in interpreting IP addresses.

Please give guidance on this.

Thank You

I have this problem too.
0 votes
Correct Answer by scottmac about 8 years 9 months ago

To add another point to the above post:

VLSM permits you to further enhance the hierarchical structure to the "IP address assignment by location" scheme.

or example, you could assign a /25 or /26 to a given building in a campus, summarize all the buildings into a /23or /24, further summarize all campuses in that region into a /20, further summarize all regions in that country into a /16, then go to the corporate Global level with everything summarized into a /8 for IS/IT resources, external vendor nets, etc.

By summarizing in that fashion, the routing tables can be kept small(er) and you gain some efficiency out of the hierarchy for both routing tables and filters/firewalls.

Using VLSM and CIDR blocks you can "rightsize" each of those hierarchical levels ... something you really couldn't do well with traditional classfull boundaries.

Good Luck

Scott

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (2 ratings)
Loading.
jcrussell Fri, 02/22/2008 - 08:24

Nobody ever gets "assigned" a private IP range, hence the term private. If an enterprise chooses to use the 10.0.0.0/8 IP block, then it will probably break it up into multiple smaller (such as /24) subnets. At all the places I have worked, they use the 10.0.0.0/8 space, and use some sort of logic to break it up. For example, the 2nd octet might be a location identifier, and the 3rd octet might be the floor number. Those subnets could be aggregated so that the main router at each location could advertise a /16 route instead of multiple /24 routes. So VLSM could be useful here, but still the size of the routing table won't be THAT large.

Now if said organization wants internet access, then the ISP will probably assign it a /28 or /29 and they will have to NAT/PAT the traffic.

Correct Answer
scottmac Fri, 02/22/2008 - 09:02

To add another point to the above post:

VLSM permits you to further enhance the hierarchical structure to the "IP address assignment by location" scheme.

or example, you could assign a /25 or /26 to a given building in a campus, summarize all the buildings into a /23or /24, further summarize all campuses in that region into a /20, further summarize all regions in that country into a /16, then go to the corporate Global level with everything summarized into a /8 for IS/IT resources, external vendor nets, etc.

By summarizing in that fashion, the routing tables can be kept small(er) and you gain some efficiency out of the hierarchy for both routing tables and filters/firewalls.

Using VLSM and CIDR blocks you can "rightsize" each of those hierarchical levels ... something you really couldn't do well with traditional classfull boundaries.

Good Luck

Scott

Actions

This Discussion