When working behind a router with NAT (Network Address Translation), traditional ARP resolution hits a fundamental limitation. Your local machine only maintains ARP entries for devices within the same broadcast domain - typically just your router's interface. Here's what happens at the protocol level:

1. Ping 192.168.1.100 (local device)
   → ARP cache shows 00:1A:2B:3C:4D:5E

2. Ping 8.8.8.8 (remote through NAT)
   → ARP cache shows router's MAC only

While you can't directly ARP devices across NAT boundaries, these methods may help:

1. Local Network Scanning

For devices on your LAN segment (not behind additional NAT):

# Linux/Unix:
arp-scan --localnet

# Windows PowerShell:
Get-NetNeighbor -AddressFamily IPv4 | Select-Object IPAddress,LinkLayerAddress

2. Router ARP Table Inspection

Many routers expose their full ARP cache via SNMP or web interface:

# SNMP walk example:
snmpwalk -v 2c -c public router_ip .1.3.6.1.2.1.4.22.1.2

3. Cross-Subnet MAC Discovery

For advanced networks with L3 switches:

# Cisco devices:
show ip arp | include 192.168.1.100

The fundamental reasons why direct remote MAC resolution fails:

• ARP operates at Layer 2 (same broadcast domain only)
• NAT rewrites source IP at Layer 3
• MAC addresses aren't preserved past the first hop

When you genuinely need MAC addresses for administration:

# Create a network map with nmap:
nmap -sn 192.168.1.0/24
nmap --script nbstat.nse 192.168.1.100

Remember that MAC addresses are only locally significant. For remote device identification, consider host-based agents or inventory systems that can report this information back to a central server.

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When working behind a NAT router, traditional ARP cache inspection only reveals the gateway's MAC address rather than the end device's hardware address. This occurs because:

• Layer 2 communication terminates at the router interface
• ARP resolution happens per network segment
• NAT modifies the packet headers

1. Network Scanner Tools

For local network discovery (Windows/Linux):

# Nmap scan example:
nmap -sn 192.168.1.0/24
arp -a | grep -i "192.168.1.100"

2. Router Admin Interface

Most consumer routers expose connected devices:

# Typical paths:
- http://routerlogin.net (Netgear)
- http://192.168.0.1 (TP-Link)
- Check "Attached Devices" section

3. SNMP Queries

For enterprise environments with SNMP enabled:

# Example using snmpwalk:
snmpwalk -v2c -c public router_ip .1.3.6.1.2.1.4.22.1.2

Important caveats to consider:

• MAC addresses aren't routable beyond local subnet
• Cloud services can't reveal remote MACs
• IPv6 neighbor discovery behaves differently

Combining multiple techniques for best results:

#!/bin/bash
# Network discovery script
ROUTER="192.168.1.1"
TARGET="192.168.1.42"

# Method 1: Direct ARP
ping -c 1 $TARGET > /dev/null
arp -n | grep $TARGET

# Method 2: Router query
curl -s "http://$ROUTER/cgi-bin/devices.ha" | grep -A5 $TARGET

For cross-platform solutions, consider Python with scapy:

from scapy.all import ARP, Ether, srp

def get_mac(ip):
    ans, _ = srp(Ether(dst="ff:ff:ff:ff:ff:ff")/ARP(pdst=ip), timeout=2)
    if ans:
        return ans[0][1].src
    return None