Networking
Explainer
ARP: How IP Finds a MAC Address
Your laptop knows the router's IP, but can't send a frame without one more address.
Quick check · tap an answer to continue
Lesson notes
Key points
- On a local network, Ethernet frames are delivered using MAC addresses, the 48-bit hardware addresses burned into network cards.
- ARP, the Address Resolution Protocol, maps a known IPv4 address to the unknown MAC address on the same subnet.
- The host broadcasts an ARP request asking who has this IP, and only the owner replies directly with its MAC.
- For destinations outside the subnet, the host ARPs for its default gateway's MAC, not the remote server's.
Definitions
- MAC address
- A 48-bit Layer 2 hardware address, e.g. 3C:52:82:1A:9F:07, used to deliver frames on one local link
Takeaway: ARP turns a local IP address into the MAC needed to send a frame.
Quick check: Host pings a server on another subnet. Whose MAC does it ARP for?
- The remote server
- The default gateway
- The DNS server
MAC addressARPRequest and replyDefault gateway
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