What is Address Resolution Protocol (ARP)?

Address Resolution Protocol (ARP) is a networking protocol used in the Internet Protocol (IP) suite to map an IP address to the corresponding physical (MAC) address on a local network. It plays a crucial role in the process of data transmission within a local area network (LAN). ARP is designed to operate at the link layer, which is the second layer of the OSI (Open Systems Interconnection) model.

Dissecting Address Resolution Protocol (ARP)

In the early 1980s, ARP was introduced as an integral part of developing Ethernet specifications. The origins of its design trace back to Xerox Corporation researchers who played a pivotal role in Ethernet and local area networking technology's early development.

ARP's creation stemmed from the necessity to bridge the divide between logical IP addressing and physical Ethernet addressing. During the rise of Ethernet networks and the ascendance of the Internet Protocol (IP) as the dominant internetworking protocol, ARP was developed. Its primary purpose was to provide a means of mapping IP addresses to MAC (Media Access Control) addresses within Ethernet networks. This protocol's operation and usage were officially documented in RFC 826 in November 1982.

How ARP works

To broadcast ARP requests to resolve the MAC address associated with a specific IP address on a local network, the device with the matching IP address needs to respond with an ARP reply containing its MAC address.

1. Data Packet Creation: Before data can be delivered, it must first be encapsulated into packets, typically using the Internet Protocol (IP) at the network layer (Layer 3 of the OSI model). These data packets include source and destination IP addresses in their headers, indicating where the data should be sent.
2. ARP Cache Lookup: When a device wants to send a data packet to a destination IP address within the local network, it consults its ARP cache (also known as the ARP table) to find the corresponding MAC address associated with the destination IP address. The ARP cache contains mappings of IP addresses to MAC addresses that the device has learned through previous ARP requests.
3. ARP Cache Hit: If the device finds a matching entry in its ARP cache, it retrieves the destination MAC address from the cache. The source MAC address (the device's own MAC address) is already known.
4. Ethernet Frame Construction: With both the source and destination MAC addresses known, the device constructs an Ethernet frame that encapsulates the IP data packet. The Ethernet frame includes the following key fields:
5. Source MAC Address: The MAC address of the sender (the device initiating the data transmission).
6. Destination MAC Address: The MAC address of the recipient (the device to which the data is being delivered).
7. Data Payload: The original IP data packet.
8. Ethernet Frame Header: Includes control information, frame type, and other Ethernet-specific details.
9. Data Transmission: Once the Ethernet frame is constructed, it is transmitted onto the local network. Ethernet frames are typically broadcasted within the local network, allowing all devices on the network to receive them.
10. Destination Device Reception: All devices on the local network receive the broadcasted Ethernet frame. However, only the device with the matching MAC address (the recipient) processes the frame. The destination device extracts the original IP data packet from the Ethernet frame.
11. IP Layer Processing: The destination device's network stack processes the IP data packet, including checking the destination IP address in the packet header. If the destination IP address matches the device's own IP address, the packet is further processed by the upper-layer protocols (e.g., TCP or UDP) and delivered to the appropriate application.

Types of ARP

The Address Resolution Protocol (ARP) has several variations and extensions to accommodate different network scenarios and needs. The main types of ARP include:

• ARP (IPv4 ARP): This is the most common type of ARP and is used for resolving IPv4 addresses to MAC addresses on Ethernet and other types of data link layers. It operates in IPv4 networks and is widely supported in Ethernet-based LANs.
• RARP (Reverse ARP): RARP is an older protocol used for the opposite purpose of ARP. It is used to discover an IP address based on a known MAC address. RARP was primarily used in diskless workstations or devices that needed an IP address assignment but had no means to configure one.
• Proxy ARP: Proxy ARP is a technique used by routers to respond to ARP requests on behalf of devices in different IP subnets. When a router receives an ARP request for a device in a different subnet, it can reply with its own MAC address and then route the traffic between the subnets. Proxy ARP can be used to enable communication between devices on separate IP subnets without the need for direct routing.
• Gratuitous ARP (GARP): Gratuitous ARP is used by a device to announce its own IP-to-MAC address mapping to the network, typically during initialization or in response to a network configuration change. It helps prevent IP address conflicts and updates the ARP caches of other devices on the network with the new or updated mapping.
• InARP (Inverse ARP): InARP is used in Frame Relay networks to map between DLCIs (Data Link Connection Identifiers) and IP addresses. It allows devices in a Frame Relay network to discover the IP address associated with a particular DLCI.
• ARP over ATM (ARP/AARP): ARP/AARP is used in Asynchronous Transfer Mode (ATM) networks to resolve IP addresses to ATM addresses. It serves a similar purpose to traditional ARP but is adapted to the specifics of ATM technology.
• NDP (Neighbor Discovery Protocol): While not strictly ARP, NDP is used in IPv6 networks to perform address resolution, router discovery, and other functions that ARP serves in IPv4. NDP includes mechanisms for discovering neighboring devices' MAC addresses and is an integral part of IPv6 networking.
• NDISC (Neighbor Discovery for IP Version 6 (IPv6)): NDISC is a component of NDP specifically used for address resolution in IPv6 networks. It is responsible for resolving IPv6 addresses to link-layer (MAC) addresses in Ethernet and other link-layer technologies.