Shifting 128 bits

[Roy Marples <roy%marples.name@localhost>]

Hi List!

I'm struggling with bit shifting, trying to implement an RFC.

Basically there is code that deals with uint128_t - an IPv6 address.
As I cannot realistically use that in dhcpcd I need something else. 
However, this is very much out of my comfort zone and I'm not good at 
handling bits!

Here is the code

uint8_t   l = 69;
uint128_t p = a_ipv6address;
uint128_t i = p << l;

uint8_t *id = a_buffer;
uint8_t d = a_length;
while (d-- > 0) {
       *id++ = i >> 120;
       i <<= NBBY;
}

Can anyone help me please?

Thanks

Roy