Implement IPv4 iterators and CIDR expansion

Problem: IPv4 Iterators and CIDR Expansion

You are implementing utilities to iterate over IPv4 addresses.

An IPv4 address is in dotted-decimal form: A.B.C.D, where each octet is an integer in [0, 255]. Conceptually, an IP can be treated as an unsigned 32-bit integer:

$(A \ll 24) + (B \ll 16) + (C \ll 8) + D$

Part 1 — Forward Iterator

Implement a Python class IPv4Iterator(start_ip: str) that iterates forward from start_ip (inclusive) up to 255.255.255.255 (inclusive).

• Must implement the Python iterator protocol: __init__ , __iter__ , __next__ .
• Each __next__() returns the next IP as a dotted string.
• When the iteration is finished, raise StopIteration .

Example:

• Input: start_ip = "192.168.0.1"
• Output sequence: "192.168.0.1" , "192.168.0.2" , ... , "255.255.255.255"

Part 2 — Reverse Iterator

Implement a reverse iterator IPv4ReverseIterator(end_ip: str) that iterates backward from end_ip (inclusive) down to 0.0.0.0 (inclusive).

Example:

• Input: end_ip = "192.168.0.255"
• Output sequence: "192.168.0.255" , "192.168.0.254" , ... , "0.0.0.0"

Part 3 — CIDR Range Expansion

Given a CIDR string like "192.168.1.0/24", compute the start IP and end IP of the CIDR range using bitwise operations, then iterate through and return all IPs in that range (inclusive).

CIDR format: base_ip/prefix_len

• base_ip is a dotted IPv4 string.
• prefix_len is an integer n in [0, 32] meaning the first n bits are fixed (network bits) and the remaining 32-n bits vary (host bits).

Requirements:

• Use bitwise masking to compute:
  • start_ip (network address)
  • end_ip (broadcast/highest address)
• Then iterate all addresses from start_ip to end_ip inclusive.

Example:

• Input: "192.168.1.0/24"
• Start: "192.168.1.0"
• End: "192.168.1.255"

Part 4 — Optimization Discussion

Discuss how you would optimize time and space complexity for the above tasks, especially for large CIDR ranges (e.g., /8 or /0).

Assumptions / Clarifications

• You may assume inputs are valid IPv4/CIDR strings unless you choose to add validation.
• Clearly define behavior at boundaries (e.g., starting at 255.255.255.255 , ending at 0.0.0.0 ).