Need for Both MAC and IP Addressing in Computer Networks

Computer networks use two complementary address schemes because they operate at different layers of the protocol stack: MAC address at the data link layer, and IP address at the network layer.

At a high level:

• MAC address is needed to move a frame within the same local link (e.g., an Ethernet LAN).
• IP address is needed to route a packet across different networks (subnets, routers, and beyond).

The interaction is commonly illustrated in the OSI model: addresses for local delivery differ from addresses for global routing. This layering is why IP alone is insufficient for reliable delivery on link technologies like Ethernet, which fundamentally transmit frames addressed by MAC.

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Why two types of addresses exist

Networks separate concerns across layers:

1. Link-layer delivery requires MAC addresses
   Link technologies like Ethernet send data in units called Ethernet frame that are delivered using MAC address. Routers/hosts therefore need a way to translate an IP destination to a MAC destination for the next hop on the local link.2

2. Network-layer routing requires IP addresses
   Routing between networks relies on IP address so that packets can traverse multiple hops and networks using routing tables and forwarding logic defined at the network layer.2

3. Different scope and lifetime of addresses

   • MAC address typically identifies an interface on a specific link and is used for local delivery semantics.
   • IP address identifies a host/service in the network topology, enabling end-to-end reachability even as devices move across networks (subject to IP configuration rules).

4. Translation layer exists (ARP) The mapping from IP address to MAC address on an Ethernet LAN is handled by ARP (Address Resolution Protocol).2

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Key roles of each address (with a mental model)

• MAC address answers: “Which interface on this local link should receive this frame?”
• IP address answers: “Which network (and ultimately which host) should this packet reach?”

This is why the system needs both:

• If you only had MAC addresses, you would lack a standardized mechanism for routing across networks.
• If you only had IP addresses, you would still need a mechanism for local link delivery, which Ethernet and many link-layer technologies do not perform using IP directly; they transmit frames using MAC addressing.2

What will happen if we use only IP address in computer networks?

If we attempt to use only IP addresses as the sole addressing mechanism, several concrete problems appear:

1. Link-layer delivery breaks on Ethernet-like networks
   Ethernet networks deliver using MAC-addressed frames. If frames were required to carry only IP addressing without MAC, then the underlying link-layer transmission mechanism would not have the required destination identifier to deliver bits to the correct interface on the same LAN.

2. No natural “next-hop mapping” without a MAC abstraction
   In real IP networking, hosts often send to the next-hop router on the local link, and then that router forwards using IP routing. This requires mapping the next-hop IP address to the next-hop link-layer address—done in practice via ARP. Removing MAC eliminates the conventional target for ARP and prevents link-layer delivery as defined by Ethernet.2

3. Routers would still need link-layer reachability mechanisms
   Even if you conceptually “keep IP,” routers still must transmit traffic over specific link technologies. Those technologies require link-layer addressing semantics. Without MAC (or an equivalent link-layer identifier), routers can’t reliably deliver packets to the proper neighbor interface on each hop.2

4. Broadcast/multicast behavior and efficiency would be disrupted
   ARP and link-layer neighbor discovery depend on local link characteristics (broadcast domain, link-layer frames). Without MAC-based discovery and frame addressing, you would need an alternative discovery/addressing protocol that effectively re-creates MAC-like functionality at the link layer—meaning “IP-only” just shifts the problem rather than removing it.2

In short: using only IP would not remove the need for link-layer identifiers; it would force link-layer delivery to either (a) become incompatible with existing frame-based network hardware, or (b) require a new mechanism that functions like MAC addresses anyway.

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Minimal “equivalence” argument

A common way to formalize the above is: IP routing needs a way to deliver traffic to the immediate neighbor on each link. That delivery is performed by the link layer using identifiers meaningful to that link (MAC on Ethernet).

Therefore, if you remove MAC and keep only IP, the network must still implement:

• a link-local identifier (or equivalent mapping),
• neighbor discovery on the local link,
• and frame delivery semantics.

That is, “IP-only” would implicitly recreate a MAC-like role—just redistributed into a different protocol.

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