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Chapter 2 – IP Addressing
1. How is VLSM (Variable Length Subnet Masking) different from the usual subnetting?
There is no difference. VLSM is simply an extension of basic subnetting, where the same A-B-C class address is subnetted by using masks of different lengths.
2. Why do I need VLSM?
VLSM provides a more efficient way of assigning IP addresses. It provides more flexibility in assigning an adequate number of hosts and subnets given a limited number of IP addresses.
3. What is the difference between CIDR and supernetting?
Classless Interdomain Routing is the mechanism that allows advertising both supernets and subnets outside of the normal bounds of a classful network number. Supernetting is a representation that allows masks that are shorter than the natural masks, hence creating supernets.
4. Is the classful model the cause of the growth in the global routing tables?
No. The growth of the routing tables is due to the fact that more and more organizations are connecting to the Internet. The classful model does not offer a solution to deal with such growth.
5. I have a network that uses older protocols such as RIP v1 and IGRP.
What issues should I consider in deciding whether to upgrade to newer protocols that support VLSM and CIDR?
If you feel 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.
6. Can I aggregate any routes in my routing table?
Only routes that are personally adminstered can be aggregated. Aggregating routes that are not an extension of your domain can create black holes.
7. If I leave my provider, can I keep my IP addresses?
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.
8. I have hosts that require Internet connectivity and others that do not. Can I use private addresses on some hosts and not others?
Yes, both private and global addresses can be used in the same network. When advertising routes to your provider, only the legal networks are advertised.
9. I need to connect to the Internet, and not all my addresses are registered. I cannot afford to renumber--what do I do?
You could always use Network Address Translation (NAT) to map your illegal address to a legal pool of addresses you get from your provider.