IPV6 Summarization explained 

128 64 32 16 8 4 2 1
An IPV6 address comprises letters and numbers and is represented like this:

0 1 2 3 4 5 6 7 8 9 a b c d e f
IT’S 128 BITS LONG==== 64 BITS= HOSTS PORTION , OTHER 64 = NETWORK PORTION. Devided in sets of 4 bits
called quartet.

WE ARE SUMMARIZING 2001:BD8:ABCD:1-5/64 :
2001:BD8:ABCD:1/64
2001:BD8:ABCD:2/64
2001:BD8:ABCD:3/64
2001:BD8:ABCD:4/64
2001:BD8:ABCD:5/64
You can clearly see that the change is taking place in the Last portion, thus:

0000.0000.00000.0000.0000.0000.0000.0000 == This is the representation of the Host portion
8 8 8 8 8 8 8 8 == 64 BITS

All the loopback have this 2001:BD8:ABCD in common which gives us 48 bits. the change takes place
in the forth quartet. use the first line to convert 1,2,3,4,5
for loopback1: 0000.0000.0000.0000.0000.0000.0000.00000001
for loopback2: 0000.0000.0000.0000.0000.0000.0000.00000010
for loopback3: 0000.0000.0000.0000.0000.0000.0000.00000011
for loopback4: 0000.0000.0000.0000.0000.0000.0000.00000100
for loopback5: 0000.0000.0000.0000.0000.0000.0000.00000101
8 8 8 8 8 8 8 5 = 61.

We’ve got here 5 commons bits.

The result is: 2001:BD8:ABCD::/61