In routing terminology, what is a stub network in an Internet Protocol routing domain?

Introduction / Context:
In routing and network design, the term stub network is used to describe a network segment that is connected to the larger internetwork through only one path. Understanding this term is important when configuring routing protocols such as Open Shortest Path First and when optimising route advertisements. This question asks you to identify which description correctly matches the concept of a stub network.

Given Data / Assumptions:

• Routers connect networks and can have multiple interfaces leading to different paths.
• Some networks are at the edge of the routing domain and have only a single way in or out.
• Stub networks do not forward transit traffic between different parts of the internetwork.
• The options distinguish between single and multiple entry and exit points.

Concept / Approach:
A stub network is one that is accessed by a single route from the perspective of the rest of the internetwork. Typically, it is connected to the rest of the network by one router interface. Traffic can enter or leave that network only via this single connection. This means the network does not serve as a transit path between two other networks; it is an endpoint. Routing protocols treat stub networks differently to reduce the size of routing tables and the amount of routing information that must be exchanged. Therefore, the correct description must emphasise a single connection providing both entry and exit.

Step-by-Step Solution:
Step 1: Identify that a stub network is defined by having only one way to reach it and to leave it. Step 2: Examine option b, which states that the network has only one entry and exit point and mentions a single router interface connecting it to the larger internetwork. Step 3: Compare this with option a, which incorrectly suggests that there is an entry but no exit, which would not describe a usable network path. Step 4: Recognise that options c and d describe networks with multiple exits or multiple connections, which are not stub networks but transit or multi homed networks. Step 5: Conclude that option b correctly matches the standard definition of a stub network.

Verification / Alternative check:
Network design literature and routing protocol documentation often refer to stub networks as networks that do not carry transit traffic and are accessible only through a single router. Examples include small branch office LANs that connect back to the main site over a single wide area network link. In practice, if that link fails, the stub network becomes isolated from the rest of the internetwork, highlighting the single entry and exit property.

Why Other Options Are Wrong:
Option a describes a network with one entry and no exit, which is not meaningful in routing terms, because any path that allows packets to enter but not leave in reverse direction would violate basic connectivity. Options c and d describe networks with multiple exits or multiple connections, which can act as transit networks or multi homed sites, not stub networks.

Common Pitfalls:
A common mistake is to think that stub networks have no routers at all or that they cannot send traffic out. In reality, the key property is a single connecting router interface to the rest of the internetwork. Another pitfall is confusing a stub network with a stub area in Open Shortest Path First, which is a related but slightly different concept dealing with how routing information is summarised and constrained within an area.

Final Answer:
A stub network is a network that has only one entry and exit point, that is, it is connected to the rest of the internetwork through a single router interface.