IPv6: Summarizing OSPFv3 addresses

This is from IPexpert volume 1 section 22. We have to summarize the following routes:

2020:100:100:2222::/64
2020:100:100:2666::/64

This is how I do it:

1) Identify the hextet ( I don't know if this is a word, I derived it from octet) where we will summarize. This is the 4th hextet. So we know our subnet will be somewhere between /48 and /64.

2) Break it down in binary:

2222 = 0010 0010 0010 0010
2666 = 0010 0110 0110 0110

Converting hex to binary is easy because we just treat each digit in the IPv6 address as a 4 bit binary number by itself. Notice how all the 2's broke down to 0010.

3) Find the common bits. I highlighted them in red below. We have 5 of them. We will use to get our mask.

2222 = 0010 0010 0010 0010
2666 = 0010 0110 0110 0110

4) Set the rest if the bits to 0 and calculate the summary, convert to hex.

summary = 0010 0000 0000 0000 = 2000

5) Calculate subnet mask by adding the common bits to the lower of our subnet range. This means we add 5 to /48 which makes /53.

So our summary address is 2020:100:100:2000::/53

In OSPFv3 we would enter:

R2(config)#ipv6 router ospf 1
R2(config-rtr)#area 1 range 2020:100:100:2000::/53

Check our other routers for the summary:

R4#show ipv6 route | inc 53
OI 2020:100:100:2000::/53 [110/129]