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A supernetwork, or supernet, is an
Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. I ...
(IP) network that is formed by aggregation of multiple networks (or
subnets A subnetwork or subnet is a logical subdivision of an IP network. Updated by RFC 6918. The practice of dividing a network into two or more networks is called subnetting. Computers that belong to the same subnet are addressed with an identica ...
) into a larger network. The new routing prefix for the aggregate network represents the constituent networks in a single
routing table 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 t ...
entry. The process of forming a supernet is called supernetting, prefix aggregation, route aggregation, or route summarization. Supernetting within the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, p ...
serves as a strategy to avoid fragmentation of the
IP address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...
space by using a hierarchical allocation system that delegates control of segments of address space to
regional Internet registries A regional Internet registry (RIR) is an organization that manages the allocation and registration of Internet number resources within a region of the world. Internet number resources include IP addresses and autonomous system (AS) numbers. ...
. 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.


Overview

In IP networking terminology, a supernet is a block of contiguous
subnetwork A subnetwork or subnet is a logical subdivision of an IP network. Updated by RFC 6918. The practice of dividing a network into two or more networks is called subnetting. Computers that belong to the same subnet are addressed with an identical ...
s 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 Classless Inter-Domain Routing (CIDR ) is a method for allocating IP addresses and for IP routing. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous classful network addressing architecture on the Internet. Its ...
(CIDR).
Interior Gateway Routing Protocol Interior Gateway Routing Protocol (IGRP) is a distance vector interior gateway protocol (IGP) developed by Cisco. It is used by routers to exchange routing data within an autonomous system. IGRP is a proprietary protocol. IGRP was created in ...
,
Exterior Gateway Protocol An exterior gateway protocol is an IP routing protocol used to exchange routing information between autonomous systems. This exchange is crucial for communications across the Internet. Notable exterior gateway protocols include Exterior Gatewa ...
and version 1 of the
Routing Information Protocol The Routing Information Protocol (RIP) is one of the oldest distance-vector routing protocols which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from so ...
(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 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 sy ...
, EIGRP, IS-IS and
Border Gateway Protocol Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. BGP is classified as a path-vector routing protocol, and it mak ...
.


Examples

A company that operates 150 accounting services in each of 50 districts has a router in each office connected with a
Frame Relay Frame Relay is a standardized wide area network (WAN) technology that specifies the physical and data link layers of digital telecommunications channels using a packet switching methodology. Originally designed for transport across Integrated Se ...
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: 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. 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 address An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...
es 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: *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.


See also

* Provider-aggregatable address space * Provider-independent address space


References

;Notes ;Bibliography *Comer, Douglas E. (2006). Internetworking with TCP/IP, 5, Prentice Hall: Upper Saddle River, NJ. {{refend


External links


The Supernetting/CIDR ChartSupernet Examples and How to Calculate Supernets
Network performance Internet terminology Routing Internet architecture