MAC addresses only need to be unique in a local broadcast domain, not globally, so re-use of MAC addresses in different networks usually isn't a problem.
The internet isn't one global broadcast domain and thus needs to be divided into many blocks of addresses assigned to different ISP's and each ISP divides his blocks into smaller blocks for different customers/services. To allow each of these smaller blocks to contain many MAC addresses you need to have the IP-space much bigger than the MAC address space.
The least significant bit (LSB) in the MAC-Address is the Individual/Group type specification. IEEE 802.3 specifies the MAC-Address like this (with LSB first):
+---------+---------+----------------+
| I/G Bit | L/G Bit | 46-Bit Address |
+---------+---------+----------------+
I/G Bit: If this bit is 0, it shall indicate that the address field contains an individual address. If this bit is 1, it shall indicate that the address field contains a group address that identifies none, one or more, or all of the stations connected to the LAN. In the Source Address field, the first bit is reserved and set to 0.
L/G Bit: The second bit shall be used to distinguish between locally or globally administered addresses. For globally administered (or U, universal) addresses, the bit is set to 0. If an address is to be assigned locally, this bit shall be set to 1. Note that for the broadcast address, this bit is also a 1.
When you convert the first byte of your address bb into binary, you get 10111011. Here the LSB is last, so the last bit in the first octet/byte is the I/G bit, which is 1. This makes your MAC address a group MAC address which your router rejects. If you use aa, you get 10101010 where the last bit is 0, making this an individual (unicast) MAC-Address.
To make your MAC-Adress unicast and also make clear that you've locally assigned it (and to prevent MAC collision) you should use an address where the I/G bit is turned off and the L/G bit is turned on. This means one of the following addresses:
X2:XX:XX:XX:XX:XX
X6:XX:XX:XX:XX:XX
XA:XX:XX:XX:XX:XX
XE:XX:XX:XX:XX:XX
You can use any hex value you want for X.
Best Answer
The only reason to clone a MAC in this context, is because the ISP limits connectivity to that registered MAC. That would typically be the router supplied by the ISP -- you'd clone it's MAC onto the router of your choosing. Since the modem is the router these days (i.e. SBG6580), and few (if any?) ISPs still restrict connectivity to a "registered MAC", it's rarely done.
Personally, I do it to keep the same IP, and not have to reset the modem if I change routers. The modem does enforce a limited number of devices behind it -- (1) for most residential settings, (4) or more in business settings.