[Dnsmasq-discuss] Why does the dnsmasq routing feature require a subnet prefix length of 64?

[renmingshuai]

When ra-only, slaac, or ra-stateless is configured in dhcp-range and the prefix len is set to a value other than 64, like this:
"dhcp-range=2000:1000:1000:1000:1000:1000::, ra-stateless,120,infinite"
the following error message is displayed:
dnsmasq: prefix length must be exactly 64 for RA subnets at line 16 of /etc/dnsmasq.conf
Why must the prefix length be 64? This may come from an RFC regulation or recommendation, but I didn't find it. Would you mind tell me the reason?
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