ARP and Other TCP/IP Protocol Support for MAC Addresses
Now-obsolete TCP/IP protocols called Reverse ARP and InARP could identify IP addresses from MAC addresses. Their functionality is part of DHCP. While the internal workings of DHCP manage both MAC and IP address data, the protocol does not allow users to access that data. A built-in feature of TCP/IP, Address Resolution Protocol, translates IP addresses to MAC addresses. ARP was not designed to translate addresses in the other direction, but its data can help in certain situations.
ARP Cache Support for MAC and IP Addresses
ARP maintains a list of both IP addresses and matching MAC addresses called the ARP cache. These caches are available on individual network adapters and also on routers. From the cache, it’s possible to derive an IP address from a MAC address; however, the mechanism is limited in many respects. Internet Protocol devices discover addresses through Internet Control Message Protocol messages, such as those triggered by the use of ping commands. Pinging a remote device from any client triggers an ARP cache update on the requesting device. On Windows and some other network operating systems, the arp command provides access to the local ARP cache. In Windows, for example, type arp -a at the Command Prompt or PowerShell to display all the entries in that computer’s ARP cache. This cache may be empty depending on how that local network is configured. At best, a client device’s ARP cache only contains entries for other computers on the LAN. Most home broadband routers allow viewing of their ARP caches through their console interface. This feature reveals both IP and MAC addresses for every device currently joined to the home network. Routers do not maintain IP-to-MAC address mappings for clients on other networks besides their own. Entries for remote devices can appear in the ARP list, but the MAC addresses shown are for the remote network’s router, not for the actual client device behind the router.
Management Software for Device Addressing on Business Networks
Larger business computer networks solve the problem of universal MAC-to-IP address mapping by installing special management software agents on their clients. These software systems, based on Simple Network Management Protocol, include a capability called network discovery. Network discovery systems forward messages out to the agent on every network device with a request for both the IP and MAC addresses of that device. The system receives and then stores the results in a default table separate from any individual ARP cache. Corporations that have full control over their private intranets use network management software to manage the client hardware that they also own. Ordinary consumer devices like phones don’t have SNMP agents installed, nor do home network routers function as SNMP consoles.
