Why are IP addresses required even though MAC addresses are globally unique?

Concept: In the OSI/TCP-IP layering model, a MAC address is a Layer 2 (Data Link) address burned into a network interface by its manufacturer -- it is globally unique but flat, meaning it carries no information about which network the device sits on. An IP address is a Layer 3 (Network) address with a hierarchical network-plus-host structure, and routers use only that network portion to decide the next hop when forwarding a packet between different networks.

Application: Because a MAC address has no network component, a router receiving a frame cannot tell from the MAC address alone which network, among many separate networks, the destination belongs to. Global uniqueness only guarantees that no two interfaces share the same MAC value; it does not organize interfaces into networks. The IP address's network portion is what lets a router look up a route and forward the packet toward the correct destination network, so IP addressing is required in addition to the MAC address even though the MAC address is already globally unique.

Checking each option against this concept:

• Routers strip MAC addresses permanently -- at each hop, a router replaces the frame's Layer 2 header with new source and destination MAC addresses for the next physical link; it re-addresses the frame at every hop rather than permanently removing MAC addressing from the network.

• IP addresses are faster -- address lookups for MAC-based forwarding (switching/ARP tables) and IP-based forwarding (routing tables) both run as hardware-assisted operations at comparable speeds; raw lookup speed is not the property that makes network-layer addressing necessary.

• MAC addresses expire periodically -- a manufacturer-assigned MAC address is burned into the interface and stays fixed for its lifetime, with no built-in expiry or renewal cycle (software-based MAC randomization for privacy is a separate, optional mechanism).

• MAC addresses cannot be used for routing across networks -- this matches the concept above: a flat, non-hierarchical Layer 2 address carries no network-membership information, so it cannot serve as the basis for forwarding across separate networks, which is exactly why the hierarchical IP address is needed alongside it.