How ARP Resolves IP to MAC Addresses
- Networking Fundamentals & OSI Model
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- Physical Layer & Transmission Media
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- Ethernet Protocol
- ARP (Address Resolution Protocol)
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The Gap Between Two Different Addressing Systems
As covered in the MAC vs IP note earlier in this project, delivering a frame on a local network requires a destination MAC address - but applications and routing decisions work in terms of IP addresses. ARP (Address Resolution Protocol) is exactly what bridges this gap.
What ARP's Job Actually Is
ARP answers one specific question: "I know this device's IP address on my local network - what is its MAC address?" This is a necessary step before a frame can actually be built and sent, since a frame's header requires a MAC address, not an IP address.
The ARP Cache: Avoiding Repeated Lookups
Once resolved, a device stores the IP-to-MAC mapping in a local ARP cache for a period of time, so it doesn't need to repeat the full resolution process for every single packet sent to the same destination - very similar in spirit to how DNS caching (covered earlier in this project) avoids repeating full domain name lookups.
ARP Cache
IP Address MAC Address
192.168.1.1 -> AA:BB:CC:11:22:33
192.168.1.20 -> AA:BB:CC:44:55:66Why ARP Only Works Within One Local Network
Because ARP relies on broadcasting a question to the local network segment (covered in the next note), it only functions within one single local network - resolving an IP address on a completely different, remote network requires routing the packet there first, where ARP would then be used locally on that destination network instead.
Why Understanding ARP Matters
ARP is a small, easy-to-overlook protocol, but it's a genuinely essential piece connecting IP-based routing decisions to the actual MAC-address-based frame delivery mechanism that every local network transmission ultimately depends on.