How to find the common supernet between two networks?

Question

Imagine I have two networks:

192.168.20.0
192.168.21.0

How can I find the common supernet to give to a router so any external router can access these two network by using only the supernet address?

Please, Please, Please! Read this question and the first answer (top to bottom) before asking any more IP questions: serverfault.com/questions/49765
Strictly speaking you have not given two networks, you have given two IPv4 addresses. Your question should have been; Given IP addresses 192.168.20.0 and 192.168.21.0, what is the minimum network possible that will include both. The answer is 192.168.20.0 /23.
A more difficult problem would have been; Given IP addresses 192.168.21.0 and 192.168.22.0, what is the minimum network possible that will include both.

Chris S · Answer 1 · 2010-08-27 13:08:11Z

Convert to binary:

1100 0000.1010 1000.0001 0100.0000 0000
1100 0000.1010 1000.0001 0101.0000 0000

look at how many bits they have in common:

XXXX XXXX.XXXX XXXX.XXXX XXX0.000000000

Count them up for CIDR notation; or convert to decimal for the subnet mask:

CIDR:192.168.20.0/23
Mask:255.255.254.0

topdog · Answer 2 · 2010-08-27 13:08:28Z

up vote 0 down vote

#ipcalc 192.168.20.0 - 192.168.21.255
deaggregate 192.168.20.0 - 192.168.21.255
192.168.20.0/23

answered Aug 27 '10 at 13:08

topdog
2,08236

feedback

Dom · Answer 3 · 2010-08-27 13:08:56Z

Work in binary, you will see the subnet which contains these two networks. Or use a mask calculator like mine http://dominique.fournier38.fr/?prog=CalculMasque&line=1 in french sorry)

3 Answers