Route Summarization Calculation Example

Networks to summarize :We are using /24

172.16.8.0
172.16.9.0
172.16.10.0
172.16.11.0
172.16.12.0
172.16.13.0
172.16.14.0
172.16.15.0
172.16.16.0
172.16.17.0
172.16.18.0
172.16.19.0

first we need addresses converted to binary format:
(in this example the changes occurs on the 3rd octet )

00001000 =8
00001001 =9
00001010 =10
00001011 =11
00001100 =12
00001101 =13
00001110 =14
00001111 =15
00010000 =16
00010000 =17
00010001 =18
00010011 =19

we need to locate the ON=1 bits at which the common pattern of digits ends
00001|000 =8
00001|001 =9
00001|010 =10
00001|011 =11
00001|100 =12
00001|101 =13
00001|110 =14
00001|111 =15
0001|0000 =16
0001|0000 =17
0001|0001 =18
0001|0011 =19

Now lets look for summary address(es) !!!

00001 = 8
0001 = 16

now let put this 2 numbers on the 3rd octet of the Ip
AHA !!! we have 2 summary addresses
172.16.8.0 and 172.16.16.0

Now we need to find out the subnet masks for this 2 Ip.
Locate the bit where the common pattern of digits ends for the 2 Ips.

00001|000 =8
00001|001 =9
00001|010 =10
00001|011 =11
00001|100 =12
00001|101 =13
00001|110 =14
00001|111 =15

000100|00 =16
000100|00 =17
000100|01 =18
000100|11 =19

The first 21 bits of the IP addresses from 172.16.8.0 through 172.16.15.0 Therefore /21
The first 22 bits of the IP addresses from 172.16.16.0 through 172.16.19.0 Therefore /22.

And we use this value as a 3rd octet in our net-mask

So :

172.16.8.0 255.255.248.0
172.16.16.0 255.255.252.0

Its our summarized routes / networks.