To successfully use Variable Length Subnet Masking (VLSM) in a routed IPv4 network, what capability must the routing protocols in use provide?

Introduction / Context:
This question evaluates your understanding of what makes a routing protocol classless and therefore compatible with Variable Length Subnet Masking. VLSM allows you to use different subnet masks within the same network address space, which greatly improves efficiency but also requires routing protocols to carry more detailed information than older classful protocols did.

Given Data / Assumptions:
- We are designing or troubleshooting a network that uses VLSM, meaning different subnet masks for different subnets of the same major network.
- Routing protocols are responsible for advertising reachable networks between routers.
- Some older routing protocols are classful and do not carry subnet mask information, while newer ones are classless and do.
- The question asks what specific capability routing protocols must have for VLSM to function correctly.

Concept / Approach:
In a classful world, routing updates only include the major network ID (such as 192.168.0.0) and rely on default masks based on the class (for example, /24 for Class C). This breaks when you use VLSM because different subnets of the same major network may have different masks. To support VLSM, a routing protocol must explicitly transmit the subnet mask or prefix length along with each route. Protocols that do this are called classless and include RIPv2, Enhanced Interior Gateway Routing Protocol, Open Shortest Path First, and Intermediate System to Intermediate System.

Step-by-Step Solution:
Step 1: Recall that VLSM means using multiple subnet masks within the same network address space to tailor subnet sizes to actual needs.Step 2: Understand that routers must be able to distinguish between these different subnets and their exact boundaries.Step 3: Recognize that this requires each routing update to include not only the network address but also the subnet mask or prefix length.Step 4: Identify that protocols which do this are described as classless, because they do not rely solely on old classful rules.Step 5: Conclude that the necessary capability is the transmission of subnet mask (prefix length) information with each route.

Verification / Alternative check:
You can verify this by comparing RIPv1 and RIPv2. RIPv1 does not send subnet masks in its updates and is therefore classful and incompatible with VLSM in many designs. RIPv2, on the other hand, includes subnet mask information, making it classless and able to handle discontiguous networks and VLSM. Similar behavior is found in OSPF and EIGRP, which always carry prefix information.

Why Other Options Are Wrong:
Support for multicast is useful for efficient update distribution but is not directly required for VLSM. Multi-protocol support for IPv4 and IPv6 is unrelated to whether subnet mask information is carried. Unequal cost load balancing is a feature of certain protocols like EIGRP but is not required for VLSM. Automatic translation between public and private addresses describes Network Address Translation, which operates at the edge of networks and is not a property of dynamic routing protocols used for VLSM.

Common Pitfalls:
Students sometimes think that any modern feature such as multicast support or advanced metrics automatically implies VLSM capability, which is not true. The core requirement is simple but specific: the protocol must carry the subnet mask or prefix length alongside the network address in routing updates. Remembering this detail helps quickly identify which protocols are classless and which are not.

Final Answer:
Routing protocols must be able to transmit subnet mask or prefix length information with each route in order to support VLSM.