Objectives
| # | Objective |
|---|
| 1 | Configure IP addresses on PC1, PC2 and PC3 — all on the same /24 subnet |
| 2 | Trigger ARP resolution — ping PC2 from PC1 and observe the broadcast/reply sequence in the terminal |
| 3 | Inspect ARP tables on all three PCs — understand what's cached and why PC3 is absent from PC1's table |
| 4 | Inspect SW1's MAC address table — see how ARP traffic populates it |
| 5 | Clear the ARP cache on PC1 and re-trigger resolution — confirm ARP runs again when the cache is empty |
| 6 | Troubleshoot: PC3 is unreachable — diagnose why ARP gets no reply and fix the root cause |
| 7 | Knowledge check — ARP mechanics, broadcast vs unicast, cache behaviour |
Addressing Plan
| Device | IP Address | Subnet Mask | MAC Address |
|---|
| PC1 | 192.168.10.10 | 255.255.255.0 | aabb.cc00.0010 |
| PC2 | 192.168.10.20 | 255.255.255.0 | aabb.cc00.0020 |
| PC3 | 192.168.10.30 | 255.255.255.0 | aabb.cc00.0030 |
| SW1 | — | — | Layer 2 only |
Key Concepts
| Concept | What it means |
|---|
| ARP Request | A broadcast frame sent to FF:FF:FF:FF:FF:FF — "Who has this IP? Tell me your MAC." Every device on the LAN receives it. |
| ARP Reply | A unicast frame sent directly back to the requester — "I have that IP. My MAC is X." Only the requester receives it. |
| ARP Cache | A local table mapping IP addresses to MACs. Entries are reused without broadcasting again, until they age out or are cleared. |
| Gratuitous ARP | A device announcing its own IP-to-MAC mapping — sent when an interface comes up, or to update stale caches. |
| ARP ageing | Cached entries expire after a timeout (typically 4 hours on Cisco IOS). Clear the cache with clear arp to force immediate re-resolution. |
Supported Commands
| Device | Command | Purpose |
|---|
| PC1/PC2/PC3 | ip <addr> <mask> | Set IP address and mask |
| PC1/PC2/PC3 | show ip | Show current IP configuration |
| PC1/PC2/PC3 | show arp | Display ARP cache table |
| PC1/PC2/PC3 | clear arp | Flush all ARP cache entries |
| PC1/PC2/PC3 | ping <ip> | Send ICMP echo — triggers ARP if needed |
| SW1 | enable | Enter privileged EXEC mode |
| SW1 | show mac address-table | See MACs learned from ARP traffic |
| SW1 | show interfaces status | Verify port status |
🔍 After your first successful ping, the ARP Analyser appears in the top bar — step through the broadcast request, unicast reply, and cache update frame-by-frame.
Any valid host IP in 192.168.10.0/24 is accepted. The suggested addresses above are a guide, not a strict requirement.