Supernet

Supernet

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A supernet is an Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...

 (IP) network that is formed from the combination of two or more networks (or subnet
Subnet
The word subnet may refer to:* In computer networks, an abbreviation for subnetwork.* In mathematics, a subnet of a net in a topological space....

s) with a common Classless Inter-Domain Routing
Classless Inter-Domain Routing
Classless Inter-Domain Routing is a method for allocating IP addresses and routing Internet Protocol packets. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of classful network design in the Internet...

 (CIDR) 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
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

 serves as a preventative strategy to avoid topological fragmentation of the IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...

 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
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

 networking terminology, a supernet is a block of contiguous subnetwork
Subnetwork
A subnetwork, or subnet, is a logically visible subdivision of an IP network. The practice of dividing a network into subnetworks is called subnetting....

s addressed as a single subnet in the larger network. Supernets always have subnet masks that are smaller than the masks of the component networks.

The size of routing tables has been rapidly increasing during the expansion of the Internet. Supernetting is the process of aggregating routes to multiple smaller networks, thus saving storage space in the routing table
Routing table
In computer networking a routing table, or Routing Information Base , is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics associated with those routes. The routing table contains information about the...

 and simplifying routing decisions. Furthermore, routing advertisements to neighboring gateways are reduced.

Supernetting in large, complex networks 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 fails. For example, if a router only advertises a summary route to the next router, then it does 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 Classless Inter-Domain Routing
Classless Inter-Domain Routing
Classless Inter-Domain Routing is a method for allocating IP addresses and routing Internet Protocol packets. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of classful network design in the Internet...

 (CIDR). Interior Gateway Routing Protocol
Interior Gateway Routing Protocol
Interior Gateway Routing Protocol is a distance vector interior routing protocol invented by Cisco. It is used by routers to exchange routing data within an autonomous system....

, Exterior Gateway Protocol
Exterior Gateway Protocol
The Exterior Gateway Protocol is a now obsolete routing protocol for the Internet originally specified in 1982 by Eric C. Rosen of Bolt, Beranek and Newman, and David L. Mills. It was first described in RFC 827 and formally specified in RFC 904...

 and version 1 of the Routing Information Protocol
Routing Information Protocol
The Routing Information Protocol is a distance-vector routing protocol, 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 the source to a destination. The maximum number of hops allowed for RIP is 15....

 (RIPv1) are based on classful addressing, and therefore cannot transmit subnet mask information.

Enhanced Interior Gateway Routing Protocol
Enhanced Interior Gateway Routing Protocol
Enhanced Interior Gateway Routing Protocol - is a Cisco proprietary routing protocol loosely based on their original IGRP. EIGRP is an advanced distance-vector routing protocol, with optimizations to minimize both the routing instability incurred after topology changes, as well as the use of...

 (EIGRP) is a classless routing protocol capable of support for CIDR. By default, EIGRP summarizes the routes within the routing table and forwards these summarized routes to its peers. This may have adverse impact within heterogeneous routing environments with discontiguous subnets.

The family of classless routing protocols are RIPv2, Open Shortest Path First
Open Shortest Path First
Open Shortest Path First is an adaptive routing protocol for Internet Protocol networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system . It is defined as OSPF Version 2 in RFC 2328 for IPv4...

, EIGRP, IS-IS and Border Gateway Protocol
Border Gateway Protocol
The Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems . It is described as a path vector 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 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.

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, 192.168.111.0. It must be assured that the missing network prefixes do not exist outside of this route.

In another example, an ISP is assigned a block of IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...

es by a regional Internet registry (RIR) of 172.1.0.0 to 172.1.255.255. The ISP might then assign subnetwork
Subnetwork
A subnetwork, or subnet, is a logically visible subdivision of an IP network. The practice of dividing a network into subnetworks is called subnetting....

s 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
Routing table
In computer networking a routing table, or Routing Information Base , is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics associated with those routes. The routing table contains information about the...

.

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