Can I use the prefix length except for 64 bits in IPv6 Router Advertisement.

Issue

• I am using the 56 bits long prefix in IPv6 router, and it's advertised as a prefix for the subnet.
• I tried to install Red Hat Enterprise Linux 6.5 on this IPv6 network, but it failed since the network interface was not configured properly.
• When I used the 64 bits long prefix, the problem doesn't occur.