Address Resolution Protocol (ARP) – Why is it important?

ARP (Address Resolution Protocol) is a fundamental networking protocol that plays a crucial role in allowing devices to communicate on a local network. In this article, we’ll dive deep into how ARP works and its importance. So whether you’re a network administrator or just someone who wants to learn more about how your devices communicate, this article is for you!

What is Address Resolution Protocol (ARP)?

Address Resolution Protocol (ARP) is a protocol that maps dynamic IP addresses to permanent physical machine addresses in a local area network (LAN). The physical machine address is also known as a media access control (MAC) address.

ARP translates 32-bit addresses to 48-bit addresses and vice versa, which is necessary because IP addresses in IP version 4 (IPv4) are 32 bits but MAC addresses are 48 bits.

ARP works between Layer 2 and Layer 3 of the Open Systems Interconnection model (OSI model). The MAC address exists in Layer 2 of the OSI model, the data link layer. The IP address exists in Layer 3, the network layer.

ARP can also be used for IP over other LAN technologies, such as token ring, Fiber Distributed Data Interface, and IP over Asynchronous Transfer Mode.

Why is it important?

Address Resolution Protocol is essential for properly functioning IP networks as it facilitates communication between devices on a local network by mapping IP addresses to physical (MAC) addresses. This mapping is necessary for data transmission at the Data Link Layer, where communication takes place using MAC addresses rather than IP addresses. Without ARP, devices would not be able to identify each other on the network, and communication would not be achievable. Additionally, ARP also helps to detect duplicate IP addresses on a network, which can cause communication issues if not resolved.

How does the Address Resolution Protocol work?

When a computer joins a local area network (LAN), it is assigned an IP address. This may be a static IP address that remains unchanging or a dynamic one assigned by a DHCP server. When a packet destined for that computer passes through a network gateway, the gateway needs to determine where to send it, which requires the MAC address. A network gateway will keep a lookup table — called the ARP cache — that lists all known IP/MAC address mappings.

If the desired mapping exists in the ARP cache, then the gateway can send the packet to its destination. However, an IP address may not exist in the cache if it is new or if it has been purged (address mappings are typically cached for only a few minutes). In these cases, the gateway needs to find out which MAC address maps to that IP address.

This is where the Address Resolution Protocol comes into the picture. A gateway will broadcast an ARP request to all computers on the LAN asking which of them is using that IP address. The computer with that IP address will send back an ARP response that provides its MAC address. When the gateway receives this ARP response, it can send the packet on to its intended destination. It will also record the IP/MAC address mapping in its ARP cache so that it can properly route packets in the future.

What are the types of ARP?

There are several types of Address Resolution Protocols, including:

• Gratuitous ARP: Gratuitous ARP is an unsolicited ARP response sent by a device to announce its IP and MAC addresses to the entire network. This helps in detecting IP address conflicts, updating ARP tables, and informing network devices about changes in hardware addresses.
• Reverse ARP: Reverse ARP (RARP) allows a device to discover its own IP address when it only knows its MAC address. This protocol is now considered obsolete, as it has been replaced by the Dynamic Host Configuration Protocol (DHCP).
• Inverse ARP: Inverse ARP is used in Frame Relay and Asynchronous Transfer Mode (ATM) networks to discover the IP address associated with a specific virtual circuit.
• Proxy ARP: Proxy ARP occurs when a router or another network device responds to ARP requests on behalf of another device, usually on a different subnet. This enables devices on separate subnets to communicate as if they were on the same network segment.

ARP vs. DHCP vs. DNS: What’s the difference?

ARP, DHCP, and DNS are all response-request protocols used on IP networks. But they serve different purposes.

• ARP maps the MAC address of a machine to its IP address on the local network. Address Resolution Protocol comes into the spotlight when you want to send a data packet to another device on the local network, but you only know its IP address, not the MAC address.
• DHCP retrieves network configuration, including IP and DNS server addresses. DHCP automatically assigns an IP address to the new device you add to the local network, so you wouldn’t need to set an IP address for each device manually. It can provide other network configuration parameters, too.
• DNS finds the IP address of a website based on its domain name or URL. For example, when you enter “nordvpn.com” into your browser’s address bar, DNS translates this human-readable domain name into IP addresses that computers can identify.
  When it comes to ARP and DHCP, we are talking about the local area network and the local (or private) IP addresses. DHCP is (usually) what assigns the devices with their IP addresses on the local network, and ARP translates those IP addresses into MAC addresses.

Meanwhile, DNS also works with global (or public) IP addresses outside the local network. DNS translates any domain name on the internet into its corresponding IP addresses.

Conclusion

Address Resolution Protocol was introduced in the early days of computer networking to solve the problem of mapping IP addresses to MAC addresses. It has since evolved and improved with the development of Proxy ARP and the introduction of security measures to protect against ARP attacks.

Today, ARP remains a fundamental network communication component, allowing devices to dynamically resolve IP addresses to MAC addresses within a local area network. It plays a critical role in facilitating efficient and reliable communication between devices on the internet. Not only is ARP important for communication, but it also improves performance, security, network reconfigurability, troubleshooting, and interoperability.

All being said, ARP is an essential protocol in enterprise networks, enabling seamless and reliable communication between devices.