--- title: "Why IP address when MAC exists" subject: "Computer Networks and Security" chapter: 4 key: "semester_7/computer_networks_and_security:mac-vs-ip-address" source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/mac-vs-ip-address site: BCT Engineering --- # Why IP address when MAC exists Computer Networks and Security · Chapter 4: Network Layer - Asked in: 2072 Chaitra, 3 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/mac-vs-ip-address - Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-4 ## Question Though we have MAC address, why do we use IP address to represent the host in networks? Explain your answer. ## Short answer MAC addresses are flat, local, and not routable; IP addresses are hierarchical and identify a host's position across networks. Routers need IP prefixes to aggregate routes, while ARP maps IP to MAC for the final local delivery. ## Answer A MAC address identifies a network interface on one local link, while an IP address identifies the host's position in the whole internetwork. Both are needed because MAC addresses cannot be used for routing. 1. **Flat, no hierarchy.** A MAC address (48 bits) is burned in by the manufacturer and has no information about location. A router would need one entry for every host in the world. An IP address has a network part and a host part, so one entry covers a whole network. 2. **Local scope only.** MAC addresses are used only inside one LAN segment; at every hop a router removes the old frame header and builds a new frame with new MAC addresses. 3. **Independent of technology.** Networks use Ethernet, Wi-Fi, PPP, etc., which have different address formats. IP gives one uniform logical address. 4. **Mobility and replacement.** If a NIC is replaced, the MAC address changes; if a host moves to another network, its MAC stays the same but its location changes. An IP address is assigned to match the network the host is on, so it can be changed as needed. 5. **Efficient routing and aggregation.** IP prefixes (for example 192.168.1.0/24) can be summarised into larger blocks. ARP maps IP to MAC for the last hop: IP finds the network, MAC delivers the frame inside it. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a MAC address unsuitable for routing across the Internet? A. It changes at every packet B. It is too long to store C. It is flat and carries no location information, so routers would need an entry per host D. It can only be used with IPv6 Answer: C. MAC addresses have no network part, so aggregation is impossible; IP addresses have network and host parts. (Key point: Flat vs hierarchical addressing) 2. What happens to MAC addresses as a packet crosses routers? A. They are replaced with new frame MAC addresses at every hop B. They stay the same end to end C. They are converted to port numbers D. They are copied into the IP header Answer: A. MAC addresses are valid only on one link; each router builds a new frame with new MACs. (Key point: Scope of MAC addresses) 3. How do IP and MAC addresses work together for delivery? A. ARP maps MAC to port numbers B. IP finds the network, MAC delivers the frame inside it, with ARP mapping IP to MAC C. MAC finds the network, IP delivers inside it D. Both are used only inside the LAN Answer: B. IP identifies the destination network and ARP resolves the MAC for the last hop. 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