ARP on a Same LAN: First Action When Host A Knows Host B’s IP but Not Its MAC
When Host A and Host B are on the same LAN, Host A must resolve Host B’s IP address to Host B’s MAC address before Host A can deliver an IP packet at the link layer. If Host A has no Host B entry in its ARP cache (an ARP cache miss), the standard first step is to send an ARP request on the LAN asking for the MAC corresponding to Host B’s IP. This ARP request is broadcast at Layer 2 (so that the correct host can hear it) and the requesting host expects an ARP reply from the target host.
Therefore, the first event among the choices is: (i) Host A broadcasts an ARP request asking for the MAC address corresponding to Host B's IP address.
type="tip" title="Key idea" content="On the same LAN, IP delivery requires a MAC address. If the MAC isn’t in the ARP cache, the host must ARP first (broadcast request → unicast reply), not send the IP packet directly."
What happens first (ARP cache miss on the same LAN)
- 1Step 1
Host A looks up Host B’s IP address in its ARP cache. Because it is not present, Host A cannot map IP_B → MAC_B.
- 2Step 2
Host A sends an ARP request frame to the LAN asking: “Who has IP_B? Tell me your MAC.”
- 3Step 3
Host B recognizes its IP and replies with an ARP reply containing its MAC address to Host A.
- 4Step 4
After updating its ARP cache, Host A encapsulates the IP packet in an Ethernet frame addressed to Host B’s MAC and transmits it on the LAN.
Matching the provided options to the correct “first” event
| Option | Statement | Is it the first event? | Why |
|---|---|---|---|
| (i) | Host A broadcasts an ARP request asking for the MAC address corresponding to Host B's IP address | ✅ Yes | It’s required to resolve IP→MAC on a LAN when the ARP cache lacks the mapping. |
| (ii) | Host A unicasts the IP packet directly to Host B | ❌ | Unicast IP transmission still requires an Ethernet destination MAC; without ARP resolution, Host A can’t form the correct frame. |
| (iii) | Host B broadcasts its MAC address | ❌ | Host B does not “broadcast its MAC”; it responds specifically to the requester via an ARP reply (typically unicast). |
| (iv) | The router assigns a new IP address to Host A | ❌ | Routers don’t assign IP addresses in response to ARP; ARP is a local link-layer address-resolution mechanism on the LAN. |
Why the router option is not first
Local delivery timeline (same LAN, ARP needed)
ARP cache lookup
t0Host A finds no mapping for Host B’s IP in its ARP cache."
ARP request
t1Host A broadcasts an ARP request for the target IP’s MAC."
ARP reply
t2Host B replies with its MAC address."
Frame transmission
t3Host A sends the Ethernet frame carrying the IP packet to Host B."
Knowledge Check
Host A knows Host B’s IP address, both are on the same LAN, and Host A does not have Host B’s MAC address in its ARP cache. What happens first?