In IP routing, what is the difference between a classful routing protocol and a classless routing protocol in terms of how they handle subnet masks and network design?

Introduction / Context:
This question tests your conceptual understanding of the difference between classful and classless routing protocols, a core topic in many Cisco certification exams. Knowing this distinction explains why older protocols struggle with modern Variable Length Subnet Masking (VLSM) designs, while newer protocols handle complex addressing schemes and discontiguous networks efficiently.

Given Data / Assumptions:
- We are working in an IPv4 routing environment.
- Some routing protocols are described as classful and others as classless.
- The focus is on how these protocols treat subnet masks in routing updates.
- We assume familiarity with examples like RIPv1 and IGRP (classful) and RIPv2, OSPF, EIGRP, and IS-IS (classless).

Concept / Approach:
Classful routing protocols do not carry subnet mask or prefix length information in their routing updates. They rely on the old notion of IPv4 address classes (A, B, and C) and assume a default mask based on the class of the network. In contrast, classless routing protocols explicitly include the subnet mask or prefix length with each advertised route. This allows classless protocols to support VLSM, discontiguous networks, and more flexible summarization strategies without being tied to class boundaries.

Step-by-Step Solution:
Step 1: Recall that classful protocols, such as RIPv1 and IGRP, never send subnet masks in routing updates and automatically summarize at classful boundaries.Step 2: Understand that this behavior prevents them from correctly representing networks that use multiple different masks for subnets of the same major network (no VLSM support).Step 3: Recall that classless protocols, such as RIPv2, EIGRP, OSPF, and IS-IS, always include subnet masks or prefix lengths in their route advertisements.Step 4: Recognize that including the mask allows routers to know the exact size of each subnet and to support VLSM and discontiguous addressing.Step 5: Match this conceptual difference to the option that explicitly mentions subnet masks and prefix lengths in updates.

Verification / Alternative check:
A practical way to verify this understanding is to think about a network that uses 192.168.1.0/24, 192.168.1.128/25, and 192.168.1.192/26. A classful protocol would have trouble representing these different masks correctly, while a classless protocol can advertise each route with its mask, making VLSM work as intended. This real design example confirms the importance of mask information in classless protocols.

Why Other Options Are Wrong:
Option B reverses the truth; it is classless protocols that support VLSM. Option C incorrectly associates classful protocols with IPv6, which is entirely classless and does not use traditional classes. Option D confuses routing algorithm types with classful or classless behavior; both distance-vector and link-state protocols can be classless. Option E ignores the clear differences in how subnet masks are handled and is therefore incorrect.

Common Pitfalls:
A common mistake is to assume that any modern feature such as authentication or triggered updates automatically means “classless,” when the key property is actually the inclusion of subnet mask information in routing updates. Another pitfall is to forget that classful behavior also implies automatic summarization at class boundaries, which can break designs with discontiguous subnets. Keeping your focus on subnet mask advertisement makes it easier to remember the distinction.

Final Answer:
Classful routing protocols do not include subnet masks in their updates and rely on default classful masks, whereas classless routing protocols always advertise routes with explicit subnet masks or prefix lengths.