Frequently Asked Questions

Q—How is VLSM different from the usual subnetting?

A—It is not. VLSM is an extension of the basic subnetting where the same class A/B/C address is subnetted by using masks of different length.

Q—Why do I need VLSM?

A—VLSM provides a more efficient way of IP address assignment. It provides more flexibility in assigning adequate numbers of hosts and subnets given a limited number of IP addresses.

Q—What is the difference between CIDR and supernetting ?

A—Classless Interdomain Routing is the mechanism that allows to advertise both supernets and subnets outside of the normal bounds of a classfull network number. Supernetting is a representation that allows masks that are shorter than the natural masks, hence creating supernets.

Q—Is the classfull model the cause of the growth in the global routing tables?

A—No. The growth of the routing tables is due to the fact that more and more organizations are connecting to the Internet. The classfull model does not offer a solution to deal with such growth.

Q—I have a network that uses older protocols such as RIP v.1 and IGRP. What issues should I consider in deciding whether to upgrade to newer protocols that support VLSM and CIDR?

A—If you feel that implementing VLSM and CIDR can help you utilize your address space more efficiently and give you better route summarization capabilities, then you should upgrade. One issue could be whether your current hardware is capable of running newer protocols that might need extra processing or memory requirements. That of course depends on the protocol to which you are upgrading. Other issues have to deal with the co-existence of new and old protocols. Because network upgrades are usually done in stages, you will be faced with situations where both older and newer protocols are running concurrently. Because older protocols cannot deal with VLSM or CIDR, you should not be surprised that extensive use of static routing might be required to ensure connectivity in your domain during the transition period.

Q—Can I aggregate any routes in my routing table?

A—You can only aggregate routes that you administer. Aggregating routes that are not an extension of your domain could create black holes.

Q—If I leave my provider, can I keep my IP addresses?

A—For the purposes of better aggregation, today's routing practices recommend (sometimes require) that you return the old addresses and get addresses from your new provider. Ask your provider for its policies.

Q—I have hosts that require Internet connectivity and others that do not. Can I use private addresses on some hosts and not others?

A—Yes, you can use both private and global addresses in the same network. When advertising routes to your provider, you only advertise the legal networks.

Q—I need to connect to the Internet, and not all my addresses are registered. I can't afford to renumber—what do I do?

A—You could always use Network Address Translation (NAT) to map your illegal address to a legal pool of addresses you get from your provider.