What is an ARP Table?

What is an ARP table and how does it work?

The ARP (Address Resolution Protocol) table is a critical component of network communication in IPv4-based networks. It functions like a phone book for your computer, mapping IP addresses (Layer 3) to MAC addresses (Layer 2) so that devices can communicate over a local network.

When a device wants to send data to another device on the same local network, it needs to know the destination’s MAC address. While it might have the IP address, that’s not enough to physically deliver the data. That’s where ARP comes in. The device sends out an ARP request—a broadcast message asking, “Who has this IP address?” The device with the matching IP replies with its MAC address. This address mapping is then stored in the ARP table to avoid asking the same question repeatedly.

The ARP table is a dynamic cache that contains rows of IP-MAC pairs. Each entry typically has three pieces of information:

• The IP address

• The corresponding MAC address

• The type of entry (dynamic or static)

Dynamic entries are created automatically whenever a device learns a new IP-MAC mapping through ARP communication. These entries have a limited lifetime and are cleared after a timeout period or when the device reboots. Static entries, on the other hand, are manually configured and persist until explicitly removed. They’re often used for devices like routers, printers, or servers that should always have the same address mappings.

The ARP table plays a vital role in efficiency. Without it, a device would need to send an ARP request every time it wanted to communicate—creating unnecessary traffic and delay. By caching this information, ARP reduces network overhead.

Importantly, ARP only works within the local subnet. If a device wants to communicate with an IP address on a different network, it sends the data to the MAC address of the local router (the default gateway), not the final destination. The router then uses its own ARP table to continue forwarding the packet.

Because ARP operates at a low level of the network stack, it’s also a potential vector for security issues. Techniques like ARP spoofing or ARP poisoning involve sending falsified ARP messages to a network, tricking devices into associating IP addresses with incorrect MAC addresses. This can be used in man-in-the-middle attacks, where an attacker intercepts or alters communications.

In summary, the ARP table is a behind-the-scenes mechanism that helps devices communicate efficiently over local networks by mapping IP addresses to MAC addresses. It’s essential for local packet delivery and, while generally reliable, it’s important for network administrators to understand how it works to troubleshoot connectivity issues and secure the network against potential misuse.

How do you view the ARP table on different operating systems?

Viewing the ARP table is an essential task for network diagnostics and management. Fortunately, every major operating system provides a way to access this information using built-in commands. Here’s how to do it across Windows, Linux, and macOS:

Windows

On Windows, you can use the arp command via the Command Prompt.

• Open Command Prompt

• Type: arp -a

• Press Enter

This will display a list of IP addresses and their corresponding MAC addresses, along with the interface they’re associated with. The output typically includes:

• Internet Address: the IP address

• Physical Address: the MAC address

• Type: either “dynamic” or “static”

You can also specify a particular interface if your machine has multiple network adapters.

Linux

On Linux, the ARP table can be viewed using several different commands, depending on the distribution and networking tools available.

• The traditional way:

  • arp -n

  • This shows the ARP table in a numeric format (without trying to resolve hostnames).

• Modern Linux distributions prefer:

  • ip neigh

  • This command provides more flexible and comprehensive output and is part of the iproute2 suite, which has replaced many older net-tools commands.

The ip neigh output will show:

• IP address

• MAC address

• Interface name (like eth0 or wlan0)

• Entry status (e.g., REACHABLE, STALE, etc.)

macOS

macOS, being UNIX-based, uses a similar syntax to Linux:

• Open Terminal

• Type: arp -a

• Press Enter

Just like in Windows, this will list IP-to-MAC address mappings for all interfaces. The format is human-readable and includes both IP and MAC addresses, though it doesn’t show the entry type explicitly (you can infer dynamic vs. static based on whether you’ve manually configured it).

Extra Tools and Notes

• Wireshark, a popular network protocol analyzer, can also show ARP traffic in real-time, allowing users to see how ARP requests and replies populate the table.

• PowerShell (Windows) can also be used with Get-NetNeighbor for similar results, especially in newer Windows environments.

Why This Matters

Being able to view the ARP table helps administrators:

• Confirm whether a device is reachable at Layer 2

• Troubleshoot IP conflicts or spoofing

• Identify unauthorized devices on the local network

Ultimately, regardless of the OS, inspecting the ARP table is a low-overhead, high-value action that offers immediate visibility into the mechanics of local network communication.

3. Why does the ARP table keep changing or clearing?

If you’ve ever looked at the ARP table and found that entries seem to appear and disappear, you’re not alone. This is normal behavior due to the way ARP caches are designed to work, but it can be confusing if you don’t understand the mechanics.

Dynamic Nature of ARP

The ARP table is essentially a cache—a temporary storage of IP-to-MAC mappings. Entries in this table are generally learned dynamically as devices communicate on the network. When your device sends out an ARP request and gets a reply, it stores that info in the ARP table—but only for a limited time.

This timeout is usually set by the operating system and varies depending on the platform. For example:

• Windows typically times out ARP entries after 2 minutes (though it can extend if used).

• Linux uses a more complex system that accounts for “reachable” and “stale” states.

• macOS may hold entries for 20 minutes or more.

Once the timeout expires, the entry is flushed from the table—unless it is used again before expiration.

Network Churn and Changes

Other reasons the ARP table may change include:

• DHCP Reassignments: Devices may receive new IP addresses after lease expiration or reboot.

• Device Reboots: When a device restarts, it might reintroduce itself with a new MAC (especially virtual machines).

• Multiple Interfaces: If a device has more than one interface (e.g., wired and wireless), ARP mappings might switch based on which interface is active.

• Gratuitous ARP: Sometimes a device sends an unsolicited ARP reply to announce its presence—this can update the table.

• Network Reconfiguration: New routers, switches, or VLAN changes can reset communication paths and ARP states.

Security Tools and Spoofing

If you’re noticing erratic changes in your ARP table, it could also be a sign of ARP spoofing or poisoning. This is when a malicious device on the network sends fake ARP messages to trick devices into associating the wrong MAC address with a trusted IP—often the gateway. This can lead to man-in-the-middle attacks or denial of service.

How to Make Entries Persistent

If you need ARP entries to remain in place—for example, for printers or static servers—you can create static entries. These do not age out or change unless removed manually. They’re configured using commands like:

• Windows: arp -s <IP> <MAC>

• Linux: ip neigh add <IP> lladdr <MAC> dev <interface> nud permanent

• macOS: sudo arp -s <IP> <MAC>

Just remember: persistent entries don’t adapt well to changes, so use them sparingly.

What is the difference between a static and dynamic ARP entry?

ARP entries come in two varieties—dynamic and static—each with different behaviors, use cases, and implications for network performance and security.

Dynamic ARP Entries

Dynamic entries are created automatically by the system when it needs to resolve an IP address to a MAC address. This happens whenever a device wants to send data to another local device but doesn’t already know its MAC address. The system broadcasts an ARP request, waits for a response, and then caches the result in the ARP table.

These entries:

• Have a limited lifetime (expire after a timeout)

• Are replaced or refreshed as needed

• Allow for flexibility, especially in environments with DHCP

This mechanism ensures that the ARP table stays current with the changing state of the network. However, the dynamic nature also introduces a potential vulnerability: these entries can be spoofed. Attackers can send false ARP replies to overwrite legitimate entries and intercept traffic.

Static ARP Entries

Static entries are manually configured by a user or administrator. They provide a fixed mapping between an IP address and a MAC address that does not change over time or due to network events.

For example, you might set a static ARP entry for a mission-critical server like this:

• Windows: arp -s 192.168.1.10 00-14-22-01-23-45

• Linux: ip neigh add 192.168.1.10 lladdr 00:14:22:01:23:45 dev eth0 nud permanent

These entries:

• Do not expire

• Cannot be overwritten by ARP replies

• Offer a layer of security, preventing spoofing (in theory)

• Require manual updating if a device changes its MAC or IP

Use Cases

• Dynamic ARP is great for environments with frequent IP or MAC address changes—like guest networks, mobile devices, and most general-purpose LANs.

• Static ARP is preferred for fixed infrastructure components: routers, firewalls, printers, or key servers. It’s also used in high-security environments to lock down known devices.

However, static ARP tables can be cumbersome to maintain. In large networks, updating static entries for every IP/MAC change becomes impractical. That’s why they’re usually reserved for niche purposes or small-scale scenarios.

Security Implications

Static entries provide protection against ARP poisoning because the system will ignore unsolicited ARP replies for known static IPs. But this isn’t foolproof—if other devices don’t use static entries or if attackers gain access to routers, your security can still be compromised. In practice, ARP security should also involve dynamic ARP inspection, MAC filtering, or even 802.1X authentication for layered defense.