Illustrated TCP/IP Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568   Pub Date: 11/01/98  

Previous Table of Contents Next


Chapter 71
Address Terms and Definitions

There are four terms used in this section:

Variable Length Subnet Masks (VLSM). The ability to place a variable-length subnet mask on a single IP network address. Refer to RFC 1817. VLSMs are explained in detail in the OSPF section.

Supernetting. A mask that is shorter than the IP network address natural mask.

Classless Inter-Domain Routing (CIDR). An advertisement mechanism that allows for advertising routes without regard to class assignment. The route could be identified by a supernet or by an extended subnet mask.

Address aggregation. The ability to summarize contiguous blocks of IP addresses as one advertisement.

The ability to manipulate IP addresses is affected not only on customer sites but within the global Internet as well. Class-oriented IP addresses are still used in the customer environment, whereas Classless IP addressing is used in the Internet itself. Customers are free to use whichever mechanism efficiently uses the address that is assigned to them. No longer are they restricted to use only one subnet mask for their assigned network number. OSPF and RIP2 gave us more flexibility when using the subnet mask. These routing update protocols distribute the subnet mask for each entry in its table. The allowed us great flexibility in mask assignment and allowed for more efficiency of the network address. For a single network ID, we could move the mask around to various masks for the single network ID. A site could make very efficient use of its assigned network ID using VLSM. We could move the mask down to 255.255.255.252 for serial lines allowing 2 bits for the host, and then move the mask around again for a various number of hosts. OSPF also allowed for summaries in the routing updates, which allowed routers to send out one network number with a mask as an update indicating all bits in the mask handled by that router. This is very efficient.

Address Terms and Definitions

•  Varible Length Subnet Masks (VLSM)—The ability to place a variable-length subnet mask on a single IP network number.
•  Supernetting—The ability to apply a mask to an IP address that is shorter than its natural mask.
•  Classless Inter-Domain Routing (CIDR)—An advertisement mechanism that allows for advertising routes without regard to Class assignment. The route could be identified by a supernet or by an extended subnet mask.
•  Address aggregation—The ability to summarize contiguous blocks of IP addresses as one advertisement.


Previous Table of Contents Next