RARP, short for “Reverse Address Resolution Protocol” or “Reverse ARP,” is a networking protocol employed by a computer to ask for its IP address from a gateway server’s Address Resolution Protocol (ARP) table or cache. Let’s simplify by looking at the tech terms individually.
A gateway server serves as a middleman between a computer and a remote server, providing additional security by hiding the remote server’s address from the computer. The ARP, meanwhile, is the communication protocol used to discover the media access control (MAC) address associated with an IP address. The ARP table lists the MAC addresses and their corresponding IP addresses.
The network administrator creates the ARP table, which gets stored in the gateway server. This table points the user to the server (identified by its MAC address) that provides the computer’s IP address.
Other interesting terms…
Read More about the “Reverse ARP (RARP)”
The University Network Group proposed RARP in 1984 so various workstations connected to a network can obtain their respective IP addresses. These workstations were diskless, which meant they couldn’t store their IP addresses. They were computers that Sun Microsystems produced at that time.
How Does Reverse ARP Work?
You should know that computers with disks don’t need to use Reverse ARP to obtain their IP addresses. But diskless systems need RARP to get identified via their IP addresses within the network. The diagram below shows how RARP works.
Your computer (host) asks a RARP server for its IP address. After that, the server replies with your IP address. Still confused? Watch this video to get a clearer picture.
RARP and ARP, What’s the Difference?
The most significant difference between the two protocols lies in what the reply a computer gets is. ARP fetches the MAC address of a remote computer, while RARP fetches the IP address of the machine used to send the query. The following table sums up all the differences between ARP and RARP.
| RARP | ARP |
| RARP stands for “Reverse Address Resolution Protocol” of “Reverse ARP.” | ARP stands for “Address Resolution Protocol.” |
| RARP lets you find your own IP address. | ARP lets you find the IP address of a different computer. |
| Your computer knows its MAC address, which the server turns into its IP address. | Your computer knows the other device’s IP address, which the server turns into its MAC address. |
| RARP uses the value “3” for requests and “4” for responses. | ARP uses the value “1” for requests and “2” for responses. |
Does Reverse ARP Usage Have Disadvantages?
The quick answer is yes. These disadvantages could be why the Bootstrap Protocol (BOOTP) and Dynamic Host Configuration Protocol (DHCP) replaced RARP. BOOTP automatically assigns an IP address to a network-connected device from a configuration server when it boots up. DHCP, meanwhile, is an enhanced version of BOOTP.
The downsides of Reverse ARP include:
- The RARP server must be part of the same physical network.
- A router can’t forward the RARP request to any other device.
- RARP can’t handle the subnetting process. Networks that are split into multiple subnets require a RARP server for each subnet.
- RARP-enabled or diskless computers can’t be configured in today’s networks.
- RARP isn’t compatible with Ethernet.
While BOOTP and DHCP have taken RARP’s place in modern-day networks, some still use RARP to handle MAC migration, particularly in virtual machines (VMs). Sample uses include:
- Cisco’s Overlay Transport Virtualization (OTV) uses RARP to update the layer 2 forwarding tables when a MAC address moves between data centers.
- VMware’s vSphere vMotion uses RARP when a VM MAC moves between hosts.
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Reverse ARP may have been designed for the computers of long ago, but as you’ve seen, even the most advanced technology still uses it to this day.
