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Supernetwork

January 01, 1970

"Supernet" redirects here. For the broadband network in the province of Alberta, see Alberta SuperNet.

A supernetwork, or supernet, is an Internet Protocol (IP) network that is formed by aggregation of multiple networks (or subnets) into a larger network. The new routing prefix for the aggregate network represents the constituent networks in a single routing table entry. The process of forming a supernet is called supernetting, prefix aggregation, route aggregation, or route summarization.

An example of route aggregation as a part of CIDR

Supernetting within the Internet serves as a strategy to avoid fragmentation of the IP address space by using a hierarchical allocation system that delegates control of segments of address space to regional Internet registries.[1] This method facilitates regional route aggregation.

The benefits of supernetting are efficiencies gained in routers in terms of memory storage of route information and processing overhead when matching routes. Supernetting, however, can introduce interoperability issues and other risks.[2]

Overview edit

In IP networking terminology, a supernet is a block of contiguous subnetworks addressed as a single subnet from the perspective of the larger network. Supernets are always larger than their component networks. Supernetting is the process of aggregating routes to multiple smaller networks, thus saving storage space in the routing table, simplifying routing decisions and reducing route advertisements to neighboring gateways. Supernetting has helped address the increasing size of routing tables as the Internet has expanded.

Supernetting in large, complex networks can isolate topology changes from other routers. This can improve the stability of the network by limiting the propagation of routing changes in the event of a network link failure. If a router only advertises a summary route to the next router, then it does not need to 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 resulting in a more stable environment.

Protocol requirements edit

Supernetting requires the use of routing protocols that support Classless Inter-Domain Routing (CIDR). Interior Gateway Routing Protocol, Exterior Gateway Protocol and version 1 of the Routing Information Protocol (RIPv1) assume classful addressing, and therefore cannot transmit the subnet mask information required for supernetting.

Enhanced Interior Gateway Routing Protocol (EIGRP) supports CIDR. By default, EIGRP summarizes the routes within the routing table and forwards these summarized routes to its peers. Other routing protocols with CIDR support include RIPv2, Open Shortest Path First, EIGRP, IS-IS and Border Gateway Protocol.

Examples edit

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 an 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 are located. These common bits are shown in red. Lastly, the number of common bits is counted. The summary route is found by setting the remaining bits to zero, as shown below. It is followed by a slash and then the number of common bits.

First Octet Second Octet Third Octet Fourth Octet Address Netmask
11000000 10101000 01100000 00000000 192.168.96.0 /20

The summarized route is 192.168.96.0/20. The subnet mask is 255.255.240.0. 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, 192.168.106.0, 192.168.107.0, 192.168.108.0, 192.168.109.0, 192.168.110.0, and 192.168.111.0. It must be assured that the missing networks do not exist outside of this route.

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, which would reduce the number of entries in the global routing table.

Risks edit

The following supernetting risks have been identified:[2]

  • Supernetting is implemented in different ways on different routers.
  • Supernetting on one router interface can influence how routes are advertised on other interfaces of the same router.
  • In the presence of supernetting, detecting a persistent routing loop becomes a difficult problem.
  • Adverse impact in heterogeneous routing environments with discontiguous subnets[3]

See also edit

References edit

Notes
  1. ^ RFC 1338, Supernetting: an Address Assignment and Aggregation Strategy, V. Fuller, T. Li, J. Yu, K. Varadhan (June 1992)
  2. ^ a b Franck Le; Geoffrey G. Xie; Hui Zhang (2011). "On Route Aggregation" (PDF). ACM. Retrieved 2013-01-10.
  3. ^ Antonio Maciá (13 February 2012). "EIGRP Summarization Issues". Retrieved 2020-07-31.
Bibliography
  • Comer, Douglas E. (2006). Internetworking with TCP/IP, 5, Prentice Hall: Upper Saddle River, NJ.

External links edit

  • The Supernetting/CIDR Chart
  • IP Address Subnetting Tutorial
  • Supernet Examples and How to Calculate Supernets

January 01, 1970
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Supernet redirects here For the broadband network in the province of Alberta see Alberta SuperNet A supernetwork or supernet is an Internet Protocol IP network that is formed by aggregation of multiple networks or subnets into a larger network The new routing prefix for the aggregate network represents the constituent networks in a single routing table entry The process of forming a supernet is called supernetting prefix aggregation route aggregation or route summarization An example of route aggregation as a part of CIDR Supernetting within the Internet serves as a strategy to avoid fragmentation of the IP address space by using a hierarchical allocation system that delegates control of segments of address space to regional Internet registries 1 This method facilitates regional route aggregation The benefits of supernetting are efficiencies gained in routers in terms of memory storage of route information and processing overhead when matching routes Supernetting however can introduce interoperability issues and other risks 2 Contents 1 Overview 2 Protocol requirements 3 Examples 4 Risks 5 See also 6 References 7 External linksOverview editIn IP networking terminology a supernet is a block of contiguous subnetworks addressed as a single subnet from the perspective of the larger network Supernets are always larger than their component networks Supernetting is the process of aggregating routes to multiple smaller networks thus saving storage space in the routing table simplifying routing decisions and reducing route advertisements to neighboring gateways Supernetting has helped address the increasing size of routing tables as the Internet has expanded Supernetting in large complex networks can isolate topology changes from other routers This can improve the stability of the network by limiting the propagation of routing changes in the event of a network link failure If a router only advertises a summary route to the next router then it does not need to 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 resulting in a more stable environment Protocol requirements editSupernetting requires the use of routing protocols that support Classless Inter Domain Routing CIDR Interior Gateway Routing Protocol Exterior Gateway Protocol and version 1 of the Routing Information Protocol RIPv1 assume classful addressing and therefore cannot transmit the subnet mask information required for supernetting Enhanced Interior Gateway Routing Protocol EIGRP supports CIDR By default EIGRP summarizes the routes within the routing table and forwards these summarized routes to its peers Other routing protocols with CIDR support include RIPv2 Open Shortest Path First EIGRP IS IS and Border Gateway Protocol Examples editA 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 an 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 0110 0010 00000000 192 168 99 0 11000000 10101000 0110 0011 00000000 192 168 100 0 11000000 10101000 0110 0100 00000000 192 168 101 0 11000000 10101000 0110 0101 00000000 192 168 102 0 11000000 10101000 0110 0110 00000000 192 168 105 0 11000000 10101000 0110 1001 00000000 Secondly the bits at which the common pattern of digits ends are located These common bits are shown in red Lastly the number of common bits is counted The summary route is found by setting the remaining bits to zero as shown below It is followed by a slash and then the number of common bits First Octet Second Octet Third Octet Fourth Octet Address Netmask 11000000 10101000 0110 0000 00000000 192 168 96 0 20 The summarized route is 192 168 96 0 20 The subnet mask is 255 255 240 0 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 0 192 168 106 0 192 168 107 0 192 168 108 0 192 168 109 0 192 168 110 0 and 192 168 111 0 It must be assured that the missing networks do not exist outside of this route 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 which would reduce the number of entries in the global routing table Risks editThe following supernetting risks have been identified 2 Supernetting is implemented in different ways on different routers Supernetting on one router interface can influence how routes are advertised on other interfaces of the same router In the presence of supernetting detecting a persistent routing loop becomes a difficult problem Adverse impact in heterogeneous routing environments with discontiguous subnets 3 See also editProvider aggregatable address space Provider independent address spaceReferences editNotes RFC 1338 Supernetting an Address Assignment and Aggregation Strategy V Fuller T Li J Yu K Varadhan June 1992 a b Franck Le Geoffrey G Xie Hui Zhang 2011 On Route Aggregation PDF ACM Retrieved 2013 01 10 Antonio Macia 13 February 2012 EIGRP Summarization Issues Retrieved 2020 07 31 Bibliography Comer Douglas E 2006 Internetworking with TCP IP 5 Prentice Hall Upper Saddle River NJ External links editThe Supernetting CIDR Chart IP Address Subnetting Tutorial Supernet Examples and How to Calculate Supernets Retrieved from https en wikipedia org w index php title Supernetwork amp oldid 1172385183, wikipedia, wiki, book, books, library,

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