Need for Two Address Schemes: IP Address vs MAC Address

A host reachable on a network needs two fundamentally different kinds of identification: a logical (software-assigned) address for locating it across the whole internet and a physical (hardware-burned) address for delivering a frame to the correct interface on the local link. The network layer uses the IP address; the data link layer uses the MAC address. They solve different problems at different scopes, and neither can replace the other.

1. What each address is

2. Why both are required

• Hierarchy vs flat identification. Routing tables would explode if every one of billions of devices had its own globally-routable entry. IP addresses are hierarchical (network + host), so a router only stores network-level routes. MAC addresses are flat and unique but carry no topological information, so they cannot be used for global routing.
• Separation of duties across layers. IP addresses are used end-to-end, from source host to destination host, by routers along the path. MAC addresses are used hop-to-hop: each router strips the incoming frame and re-frames the packet with the MAC addresses of the next hop. This layering keeps L2 LAN technologies (Ethernet, Wi-Fi) independent of L3 internetworking.
• Interoperability of heterogeneous networks. An IP datagram can cross Ethernet, token ring, ATM or Wi-Fi links because the logical address stays constant while the link-layer (MAC) address is rewritten at every hop.
• Dynamic reconfiguration. A laptop moving from Wi-Fi to Ethernet keeps its IP configuration (via DHCP) but gets a new link-layer context; the ARP mechanism binds the stable logical address to the current physical one on demand.

3. What happens if only the IP address is used

1. No local delivery mechanism. IP alone cannot put bits on a wire; frames need a MAC (or equivalent) address to be accepted by a NIC, so the last hop of delivery would be impossible.
2. Every device must store a global routing table. With a flat, non-hierarchical scheme each node would need an entry for all other nodes — $O(N)$ state per device for $N$ hosts, infeasible at internet scale.
3. Broadcast storm / scanning problem. Locating a host would require broadcasting to every device on every network, wasting bandwidth and destroying scalability.
4. Loss of technology independence. LAN hardware would have to understand IP directly, tightly coupling L2 and L3 and making new link technologies impossible to add transparently.
5. No bootstrapping. At power-on a host knows its own MAC but not yet its IP; protocols like ARP map IP → MAC precisely because both addresses must coexist.

Conclusion: the IP address answers "which network is the destination on, and how do I route there?", while the MAC address answers "which interface on this link should receive the frame?". Both are indispensable for scalable, layered, technology-independent communication.