IP Addresses, MAC Addresses and ARP

Asha's laptop has two addresses at once. One is printed in its network settings as 192.168.10.57; the other is baked into its network card as 3c:22:fb:1a:2b:3c. When she prints to 192.168.10.30, her laptop needs both kinds, and a small protocol called ARP joins them together.

The rule

IP says where on the map; MAC says which device on this wire; ARP links them.

IP address — a logical address given to a host by the network it joins, such as 192.168.10.57. It has structure: part of it names the network (Kite Studio's LAN is 192.168.10.x) and part names the host. Move the laptop to a café and it gets a different IP address, because it is now on a different network. IP addresses are what routers use to steer packets across networks.

MAC address — a 48-bit hardware address written as six pairs of hex digits, such as 3c:22:fb:1a:2b:3c. It is assigned to the network card by its maker; the first half identifies the manufacturer. It stays the same wherever the laptop goes (modern phones and laptops may deliberately randomise it on Wi-Fi for privacy). MAC addresses are what switches and network cards use to deliver frames on one local link.

ARP (Address Resolution Protocol) — the bridge between the two on an IPv4 LAN. A host knows the IP it wants to reach but needs the matching MAC to build a frame. So it broadcasts a question to every device on the LAN — "Who has 192.168.10.30? Tell 192.168.10.57." — and the owner replies directly with its MAC. The answer is kept for a while in an ARP cache, so the question is not repeated for every packet.

The shortcut

Ask one question first: "Is the destination on my own network?"

• Yes → ARP for the destination's own MAC.
• No → ARP for the default gateway's (the router's) MAC, and hand the packet to the router.

The trap is expecting ARP to find a server on another network. ARP questions are broadcasts, and broadcasts never leave the local network. Beyond the router, only IP addresses matter.

Drills

Answer fast — trust the rule.

Remember

• IP says where on the map; MAC says which device on this wire; ARP links them.
• Same network → ARP for the destination; different network → ARP for the gateway.
• ARP broadcasts stay on the LAN; they never cross a router.