Supernetwork
Template:Distinguish2 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.
The primary benefit of supernetting is the efficiency 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. dikhed
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
Example 1
Take, for instance, a class B mask of 255.255.0.0. If one borrows 2 network bits, the mask changes to 255.252.0.0; this is called supernetting. If on the other hand one were to borrow two host bits, the mask would change to 255.255.192.0, and this is called subnetting.
Example 2
As an example of how supernetting can be used as a powerful tool in a networking environment imagine a company that operates 150 accounting services in each of the 50 states and each accounting office has a router and frame relay link connected to its corporate office. Without supernetting, the routing table on any given router would have to maintain 150 routers in each of the 50 states, or 7,500 different networks. However, if supernetting is implemented, then each state would have a centralized site to connect it with all other offices. Since each router is summarized before being advertised to other states, then every router will only see its own subnets and 49 summarized entries representing other states. This would create less stress on the router’s CPU, memory, and bandwidth.
In order to determine the summary route on a router, one must first decide the number of highest-order bits that match in all addresses. See the following example which shows the process of calculating a summary route.
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
First of all, the addresses must be converted to binary format and aligned in a list as shown in the table below.
| 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 |
Second, the bits where 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, since this summarized route also contains networks that were not in the summarized group (the group was not contiguous), it must be assured that the missing network prefixes do not exist outside of this route.
For supernetting to work properly, multiple IP addresses must share the same highest-order bits and should only be implemented within classless routing protocols such as EIGRP, OSPF, RIP v.2, IS-IS for IP, and BGP.
In some cases, this feature may not be feasible. For example, RIP v.1 is a classful routing protocol that automatically summarizes based on class when advertising across a major network boundary. Automatic supernetting can potentially cause problems if summarization occurs at more than one point in the network since the summarized routes may be in conflict. When this occurs, a router receives identical summary routes from different directions. This can lead to serious connectivity issues.
Example 3
An ISP is assigned a block of IP addresses by a regional Internet registry (RIR); for example they may receive the address range of 172.1.0.0 to 172.1.255.255. They could then assign subnets to each of their downstream providers, 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.
Comer, Douglas E. (2006). Internetworking with TCP/IP, 5, Prentice Hall: Upper Saddle River, NJ.
References
External links
These are the few ones out there.