Supernetwork

Last updated August 27, 2023

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.

Overview

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

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

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
192.168.98.0	11000000	10101000	01100010
192.168.99.0	11000000	10101000	01100011
192.168.100.0	11000000	10101000	01100100
192.168.101.0	11000000	10101000	01100101
192.168.102.0	11000000	10101000	01100110
192.168.105.0	11000000	10101000	01101001

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

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]

Related Research Articles

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Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous system (AS).

In computer networking, a routing table, or routing information base (RIB), is a data table stored in a router or a network host that lists the routes to particular network destinations, and in some cases, metrics (distances) associated with those routes. The routing table contains information about the topology of the network immediately around it.

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol that is used on a computer network for automating routing decisions and configuration. The protocol was designed by Cisco Systems as a proprietary protocol, available only on Cisco routers. In 2013 Cisco decided to allow other vendors freely implement limited version of EIGRP with some of its associated features such as High Availability (HA), while withholding other EIGRP features such as EIGRP stub, needed for DMVPN and large-scale campus deployment, exclusively for themselves. Information needed for implementation was published with informational status as RFC 7868 in 2016, which did not make it into an Internet Standards Track specification and allowed Cisco to retain control of the EIGRP protocol.

A subnetwork or subnet is a logical subdivision of an IP network. The practice of dividing a network into two or more networks is called subnetting.

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Longest prefix match refers to an algorithm used by routers in Internet Protocol (IP) networking to select an entry from a routing table.

A default gateway is the node in a computer network using the Internet protocol suite that serves as the forwarding host (router) to other networks when no other route specification matches the destination IP address of a packet.

In computer networking, the default route is a configuration of the Internet Protocol (IP) that establishes a forwarding rule for packets when no specific address of a next-hop host is available from the routing table or other routing mechanisms.

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anoNet is a decentralized friend-to-friend network built using VPNs and software BGP routers. anoNet works by making it difficult to learn the identities of others on the network allowing them to anonymously host IPv4 and IPv6 services.

IP networks are divided logically into subnetworks. Computers in the same subnetwork have the same address prefix. For example, in a typical home network with legacy Internet Protocol version 4, the network prefix would be something like 192.168.1.0/24, as expressed in CIDR notation.

In computing, route is a command used to view and manipulate the IP routing table in Unix-like and Microsoft Windows operating systems and also in IBM OS/2 and ReactOS. Manual manipulation of the routing table is characteristic of static routing.

IP routing is the application of routing methodologies to IP networks. This involves not only protocols and technologies but includes the policies of the worldwide organization and configuration of Internet infrastructure. In each IP network node, IP routing involves the determination of a suitable path for a network packet from a source to its destination in an IP network. The process uses static configuration rules or dynamically obtained from routing protocols to select specific packet forwarding methods to direct traffic to the next available intermediate network node one hop closer to the desired final destination, a total path potentially spanning multiple computer networks.

References

Notes

↑ RFC 1338, Supernetting: an Address Assignment and Aggregation Strategy, V. Fuller, T. Li, J. Yu, K. Varadhan (June 1992)
1 2 Franck Le; Geoffrey G. Xie; Hui Zhang (2011). "On Route Aggregation" (PDF). ACM . Retrieved 2013-01-10.
↑ 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

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[1] RFC 1338, Supernetting: an Address Assignment and Aggregation Strategy, V. Fuller, T. Li, J. Yu, K. Varadhan (June 1992)

[Le2011-2] 1 2 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.

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