using System.Net;
using System.Net.NetworkInformation;

/// <summary>
/// Holds utilities for working with networks, Ethernet, etc.
/// </summary>
public static class NetworkUtils
{
    // http://www.codeproject.com/KB/IP/host_info_within_network.aspx
    [System.Runtime.InteropServices.DllImport("iphlpapi.dll", ExactSpelling = true)]
    static extern int SendARP(int DestIP, int SrcIP, byte[] pMacAddr, ref int PhyAddrLen);

    /// <summary>
    /// Gets the MAC address (<see cref="PhysicalAddress"/>) associated with the specified IP.
    /// </summary>
    /// <param name="ipAddress">The remote IP address.</param>
    /// <returns>The remote machine's MAC address.</returns>
    public static PhysicalAddress GetMacAddress(IPAddress ipAddress)
    {
        const int MacAddressLength = 6;
        int length = MacAddressLength;
        var macBytes = new byte[MacAddressLength];
        SendARP(BitConverter.ToInt32(ipAddress.GetAddressBytes(), 0), 0, macBytes, ref length);
        return new PhysicalAddress(macBytes);
    }
}


[TestClass()]
public class NetworkUtilsTests
{
    [TestMethod()]
    public void GetMacAddress_BroadcastIP_NonzeroMac()
    {
        IPAddress ipAddress = IPAddress.Broadcast;

        PhysicalAddress actual = NetworkUtils.GetMacAddress(ipAddress);
        Console.WriteLine(actual.ToString());
        
        Assert.AreNotEqual(PhysicalAddress.None, actual);
    }
}

http://www.codeproject.com/KB/IP/host_info_within_network.aspx

This is about the simplest way I've found. See [this](http://stackoverflow.com/questions/1092463/getting-the-mac-address-of-the-remote-host) and [this](http://stackoverflow.com/questions/1092379/want-to-get-mac-address-of-remote-pc/1092392#1092392) at StackOverflow for more info.

Get the MAC (Physical) address from a device at the specified IP address

mac

C#