In an Ethernet or IP network, how does a MAC (Layer 2) address differ from an IP (Layer 3) address, and when is each type of address used?

Introduction / Context:
MAC addresses and IP addresses are both essential identifiers in modern networks, but they operate at different layers and serve different roles. Cisco certification exams frequently ask you to distinguish between these two types of addresses and to explain how they are used together for data delivery.

Given Data / Assumptions:

• A MAC address is associated with a network interface card on a host or router.
• An IP address is configured either manually or via DHCP on a host or router interface.
• We are working within the OSI model, where MAC addresses are at Layer 2 and IP addresses are at Layer 3.

Concept / Approach:
A MAC address is a Data Link layer identifier that is used for local delivery of frames on a single broadcast domain such as an Ethernet LAN. Switches learn MAC addresses and forward frames based on them. IP addresses are Network layer addresses used for logical addressing and routing across multiple networks. Routers ignore MAC addresses beyond the local link; they forward packets based on IP destination addresses and rewrite MAC headers as packets move from one Layer 2 segment to the next.

Step-by-Step Solution:
Step 1: When one host sends data to another on the same LAN, it uses the destination IP address to decide where to send and uses ARP to resolve that IP to a MAC address.Step 2: The host constructs an Ethernet frame with the destination MAC address and a payload containing an IP packet whose destination IP address matches the remote host.Step 3: Switches in the LAN forward the frame based on the MAC address table, ensuring the frame reaches the correct port.Step 4: When traffic crosses a router to another network, the router examines the destination IP, chooses a next-hop, and rewrites the Layer 2 header, including source and destination MAC addresses appropriate for the outgoing interface.Step 5: Throughout the path, the IP address remains consistent and is used by all routers for end-to-end forwarding, while the MAC addresses change at each Layer 2 hop.

Verification / Alternative check:
By capturing packets with a tool such as Wireshark, you can see that as a packet traverses multiple hops, the IP source and destination remain constant, but the Ethernet source and destination addresses change at each router. This demonstrates the different scopes and purposes of MAC and IP addresses.

Why Other Options Are Wrong:
Option B reverses the roles of MAC and IP addresses and is factually incorrect.Option C claims that both operate only at Layer 2 and are interchangeable, which contradicts the OSI model and real network behavior.Option D incorrectly associates MAC addresses only with wireless and IP addresses only with wired networks, which is not true; both are used in wired and wireless environments.

Common Pitfalls:
Students sometimes assume that devices use IP addresses at every step, but switches actually forward based on MAC addresses. Others think MAC addresses never change, yet routers deliberately change Layer 2 headers at each hop. Keeping the distinction between local frame delivery and end-to-end packet routing clear helps avoid such confusion.

Final Answer:
The correct explanation is that A MAC address is a physical Layer 2 identifier burned into or assigned to a network interface and is used for local frame delivery on a single segment, while an IP address is a logical Layer 3 address used by routers to deliver packets end to end across multiple networks.