Runtime memory allocation in ISC DHCP differs between the IPv4 and IPv6 servers.
The DHCPv4 server will allocate memory structures at startup for each address in a network defined in dhcpd.conf. In most configurations this is not a problem but it can result in unexpected memory usage when defining a network containing millions of addresses (for example a /8.)
Because very large, sparse subnets are much more common in IPv6, the DHCPv6 server does not pre-allocate memory for every address in a subnet, only those that are used.