What is classful vs classless routing and how do DV protocols handle subnet masks?

• A. Distance vector protocols always carry masks; link-state never carries masks.
• B. Classful protocols carry masks; classless protocols carry no masks.
• C. Classful protocols don’t advertise masks; classless protocols carry subnet masks in updates; modern DV protocols typically support classless.
• D. Classful protocols always include MD5 authentication for masks.

Answer: (C)

The key idea here is how subnet mask information is conveyed in routing updates. In classful routing, updates don’t carry any subnet mask; routers apply the default mask for the address class (A, B, or C). That means you can’t distinguish subnets within the same classful network from the update alone, which makes subnetting beyond the classful boundary (like VLSM) difficult or impossible and can lead to routing inefficiencies or errors.

With classless routing, each route in the update includes the exact subnet mask (the prefix length). Routers therefore know precisely which subnets exist and how large they are, enabling CIDR, VLSM, and support for networks that aren’t neatly aligned to classful boundaries. Many modern distance-vector protocols support this classless behavior by carrying masks in their updates, which is why they can handle subnetting and discontiguous networks effectively.

So the best explanation is that classful protocols don’t advertise masks, classless protocols carry subnet masks in updates, and modern distance-vector protocols typically support classless operation.