Implement an IPv4 Iterator

Implement an iterator over IPv4 addresses. Start with a basic version: 1. Given a start IPv4 address and an end IPv4 address, support forward iteration over every address in the inclusive range. 2. Extend the iterator to support backward iteration as well. 3. Extend the input format so the iterator can be created from a CIDR block such as `192.168.1.0/24`, and still support both forward and backward traversal. 4. Discuss or implement optimizations to improve time and space complexity, especially avoiding materializing all IP addresses in memory. You may assume valid IPv4 strings in dotted-decimal notation. Be prepared to explain the core data representation, iterator APIs such as `hasNext`, `next`, `hasPrev`, and `prev`, and edge cases around boundaries.