Supernetting

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Under Supernetting refers to the combining of multiple networks with partially the same network portion into a single route. The underlying technology is the opposite of subnetting and basically means a method for addressing a larger number of hosts within an IP network . Supernetting works by reducing the network portion and the resulting increase in the host portion of an IP address. As a rule, nothing changes for the hosts, only routing entries are saved in the router by using common features in the network portion of actually different networks.

Supernetting was described in RFC 1338 and was a transition technology that was replaced in 1993 by CIDR ( RFC 1519 ), which no longer uses network classes. The possibility of combining several networks into one route by means of supernetting is called route aggregation at CIDR .

Examples

In a class C network with the associated standard network mask 255.255.255.0, 254 available host addresses are available. If more addresses are required in a network, either a class B network (subnet mask 255.255.0.0) with 65536 possible addresses must be used or the size of the network must be changed using supernetting.

Standard subnet mask network class C
Decimal 255 255 255 0
Binary 11111111 11111111 11111111 00000000
Network share Host share

In the table above, the netmask is shown in decimal and binary notation. A distinction is made between the network and host components. The 8 bits of the host part offer 256 (2 ^ 8) possible IP addresses, which, minus the network and broadcast addresses, are limited to a total of 254 addresses to be assigned for hosts.

If, for example, 1000 hosts are to be accessible without a router within a class C network with the network address 192.168.176.0, the network portion of the subnet mask must be reduced depending on the number of hosts. With 10 bits it is possible to reach 1024 (2 ^ 10) hosts.

Supernetting of a class C network
Decimal 255 255 252 0
Binary 11111111 11111111 111111 00 00000000
Network share Host share

The host part was expanded by 2 bits. The lowest IP address that can be assigned in the network is

192.168.176.1

11000000.10101000.101100 00 .00000001

and the highest

192.168.179.254

11000000.10101000.101100 11 .11111110

This form of supernetting works independently of the above example in all network classes and with greater or lesser reduction in the network share.

Web links

  • RFC 1338 - Supernetting: an Address Assignment and Aggregation Strategy