IP subnetting rule of thumb: when multiple network segments are interconnected by routers, must each routed segment (subnet) have a unique network/subnet address to ensure correct forwarding?

Introduction / Context:
IP routing depends on unique network identifiers so that routers can determine which interface to use for forwarding. Overlapping or duplicate subnet addresses cause ambiguous routes and traffic black holes. This principle applies equally to private and public addressing, and regardless of the routing protocol in use.

Given Data / Assumptions:

• Multiple network segments are connected via routers.
• Each segment is intended to be a distinct broadcast domain.
• Routing tables map destination prefixes to next hops/interfaces.

Concept / Approach:
Each routed segment must have a unique prefix (network/subnet address). Routers build forwarding entries keyed by these unique prefixes. If two segments share the same subnet address, a router cannot uniquely map destinations to the correct interface, resulting in misdelivery or dropped packets. This rule holds whether you use static routes, RIP, OSPF, EIGRP, or BGP, and whether the addresses are from private (RFC 1918) or public ranges.

Step-by-Step Solution:

Verification / Alternative check:
Labs demonstrate that overlapping subnets prevent correct ARP or route selection on multi-interface routers; renumbering to unique subnets immediately resolves the issue, validating the uniqueness requirement.

Why Other Options Are Wrong:

• Incorrect: contradicts fundamental IP routing behavior.
• Only for public ranges: uniqueness is required internally as well.
• Only with static routes/OSPF: protocol choice does not remove the need for unique prefixes.

Common Pitfalls:
Accidentally reusing subnets across VPNs without NAT; split-horizon problems masked by NAT; forgetting to update DHCP scopes after renumbering.

Final Answer:
Correct