Supernet

A supernet is an Internet Protocol (IP) network that is formed from the combination of two or more networks (or subnets) with a common Classless Inter-Domain Routing (CIDR) routing prefix. The new routing prefix for the combined network aggregates the prefixes of the constituent networks. It must not contain other prefixes of networks that do not lie in the same routing path. The process of forming a supernet is often called supernetting, route aggregation, or route summarization.

Supernetting within the Internet serves as a preventative strategy to avoid topological fragmentation of the IP address space by using a hierarchical allocation system that delegates control of segments of address space to regional network service providers. This method facilitates regional route aggregation.

The benefits of supernetting are conservation of address space and efficiencies gained in routers in terms of memory storage of route information and processing overhead when matching routes.

Overview

In Internet networking terminology, a supernet is a block of contiguous subnetworks addressed as a single subnet. Supernets always have masks that are smaller than the masks of the component networks.

Supernetting alleviates some of the issues, such as excessively large route tables which increase router latency, with the original classful addressing scheme for IP addresses by allowing multiple networks address ranges to be combined, either to create a single larger network, or just for route aggregation to keep the "Internet Routing Table" (or any routing table) from growing too large.

Supernetting refers to the process of aggregating multiple routes of Internet-connected routers, thus saving space in the routing table and speeding up packet routing. An analogy would be on a U.S. interstate highway, where a single sign points in the direction of three to five major cities. As you draw nearer to your destination, the signs start separating for the distinct paths to each city. The same principle can be applied to supernetting .

Supernetting combines a group of routes into a single route advertisement. The number of subnets and network addresses contained in Internet routing tables is rapidly increasing due to the rapid expansion of the Internet. This growth has had a negative impact on CPU resources, bandwidth, and memory used to maintain routing tables. Therefore, route summarization was introduced to reduce the size of network routing tables.

If configured properly, supernetting can reduce the latency associated with router hop, since the average speed for routing table lookup will be increased due to the reduced number of entries. The overhead for routing protocols can also be reduced since fewer routing entries are being advertised.

Another advantage of using supernetting in large, complex networks is that it can isolate topology changes from other routers. This can aid in improving the stability of the network by limiting the propagation of routing traffic after a network link goes down. For example, if a router only advertises a summary route to the next router hop, then it will not advertise any changes to specific subnets within the summarized range. This can significantly reduce any unnecessary routing updates following a topology change. Hence, it increases the speed of convergence and allows for a more stable environment.

Protocol requirements

Supernetting requires the use of routing protocols that support variable length subnet masking (VLSM) and the Classless Inter-Domain Routing (CIDR) method.

The older RIPv1 (or EGP for Exterior Routing) protocol only understands classful addressing, and therefore cannot transmit subnet mask information.

EIGRP is also a classless routing protocol capable of support for CIDR or VLSM. By default, EIGRP will summarize the routes within the routing table and forward these summarized routes to its peers. This can be disastrous within heterogeneous routing environments if VLSM has been used with discontiguous subnets and therefore auto-summarization should be disabled unless VLSM has been carefully designed and implemented.

The family of classfull routing protocols are RIPv1, and IGRP - these protocols cannot support CIDR as they do not have the ability to include subnet information within the routing updates.

The family of classless routing protocols are RIPv2, OSPF, EIGRP and BGP. EIGRP can handle multiple routed protocols such as IPX and Appletalk..

Examples

A company that operates 150 accounting services in each of 50 districts has a router in each office connected with a frame relay link to its corporate headquarters. Without supernetting, the routing table on any given router might have to account for 150 routers in each of the 50 districts, or 7500 different networks. However, if a hierarchical addressing system is implemented with supernetting, then each district has a centralized site as interconnection point. Each route is summarized before being advertised to other districts. Each router now only recognizes its own subnet and the other 49 summarized routes.

The determination of the summary route on a router involves the recognition of the number of highest-order bits that match all addresses. The summary route is calculated as follows. A router has the following networks in its routing table:

 192.168.98.0
 192.168.99.0
 192.168.100.0
 192.168.101.0
 192.168.102.0
 192.168.105.0

Firstly, the addresses are converted to binary format and aligned in a list:

Address	First Octet	Second Octet	Third Octet	Fourth Octet
192.168.98.0	11000000	10101000	01100010	00000000
192.168.99.0	11000000	10101000	01100011	00000000
192.168.100.0	11000000	10101000	01100100	00000000
192.168.101.0	11000000	10101000	01100101	00000000
192.168.102.0	11000000	10101000	01100110	00000000
192.168.105.0	11000000	10101000	01101001	00000000

Secondly, the bits at which the common pattern of digits ends (those in red, specifically the on-bits; 1s) are located. Lastly, the number of common bits is counted. The summary route should be the lowest IP address, followed by a slash, followed by the number of common bits.

The summarized route is 192.168.96.0/20. The subnet mask is 255.255.240.0.

However, this summarized route also contains networks that were not in the summarized group, namely, 192.168.96.0, 192.168.97.0, 192.168.103.0, 192.168.104.0. It must be assured that the missing network prefixes do not exist outside of this route. The summarized route may be modified to 192.168.98.0/20 to exclude the first two networks, such that the first actually routed network is specified.

In another example, an ISP is assigned a block of IP addresses by a regional Internet registry (RIR) of 172.1.0.0 to 172.1.255.255. The ISP might then assign subnetworks to each of their downstream clients, e.g., Customer A will have the range 172.1.1.0 to 172.1.1.255, Customer B would receive the range 172.1.2.0 to 172.1.2.255 and Customer C would receive the range 172.1.3.0 to 172.1.3.255, and so on. Instead of an entry for each of the subnets 172.1.1.x and 172.1.2.x, etc, the ISP could aggregate the entire 172.1.x.x address range and advertise the network 172.1.0.0/16 on the Internet community, which would reduce the number of entries in the global routing table.

References

• Comer, Douglas E. (2006). Internetworking with TCP/IP, 5, Prentice Hall: Upper Saddle River, NJ.

External links

• IP Address Subnetting Tutorial (includes supernetting)
• The Supernetting/CIDR Chart
• IP Address Subnetting Tutorial