In a typical IP based network, which device primarily examines the IP address of a packet to decide where to forward it next?

Introduction / Context:
Several different devices participate in moving data across networks, including modems, switches, routers, and hubs. Understanding which device operates at which layer and which header fields it examines is necessary for both exam questions and real world troubleshooting. This question asks you to identify the device that primarily examines the IP address of a packet when making forwarding decisions.

Given Data / Assumptions:

• Packets carry IP addresses in their headers.
• Routers operate at the network layer of the OSI model.
• Other devices such as hubs work at lower layers and do not inspect IP addresses.

Concept / Approach:
Routers are network layer devices that use routing tables to determine the best path for IP packets. When a router receives a packet, it looks at the destination IP address in the header, consults its routing table, and forwards the packet to the appropriate next hop. In contrast, hubs are simple physical layer devices that simply repeat signals to all ports without examining headers. Modems handle conversion between digital signals and analog or different physical mediums, not routing decisions. The CPU in an end system may examine IP addresses for local processing, but the key network forwarding decisions in transit are made by routers.

Step-by-Step Solution:
Step 1: Identify the network layer in the OSI model as the place where IP operates.Step 2: Recall that routers are network layer devices responsible for forwarding packets between different networks.Step 3: Recognize that hubs operate at the physical layer and do not read or understand IP headers.Step 4: Remember that modems convert between different transmission formats but do not make complex routing decisions based on IP addresses.Step 5: Choose router as the device that primarily examines IP addresses to decide the next hop.

Verification / Alternative check:
Real world network diagrams show routers at the boundaries between local area networks and wide area networks. Configuration commands for routers involve setting up routing tables, static routes, or dynamic routing protocols. Packet capture tools display how routers change the link layer headers while keeping the IP header mostly intact as the packet passes through multiple networks. This behavior confirms that routers focus on IP addresses for forwarding decisions.

Why Other Options Are Wrong:
Option A: A modem prepares signals for transmission over telephone lines or other media but does not perform IP based routing.Option C: A hub simply broadcasts incoming frames to all ports and does not inspect IP addresses.Option D: While the CPU of an end device processes packets destined for that host, it does not act as the primary forwarding device for transit traffic in the general case.

Common Pitfalls:
Students sometimes assume that any device that connects networks must be a router. In reality, switches operate at the data link layer using MAC addresses, while routers operate at the network layer using IP addresses. Mixing up these roles can lead to confusion when analyzing network problems. Focusing on which header field is examined helps keep the functions clear.

Final Answer:
The correct answer is Router.