A node id whose top bits bits are 1 and the rest 0. Port of node_id generate_prefix_mask(int bits). bits must be in 0..160.
Reproduces the upstream loop exactly:
for (; b < bits - 7; b += 8) mask[b/8] |= 0xff;
if (bits < 160) mask[b/8] |= (0xff << (8 - (bits & 7))) & 0xff;