Supernet - Examples

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

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