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What Is ARP Spoofing?

ARP spoofing is a man-in-the-middle attack against the address resolution protocol, which is how devices on a local network map an IP address to a physical address. The attacker floods the network with forged ARP replies so their own machine receives traffic meant for someone else, silently intercepting and altering it. Prevent it with dynamic ARP inspection on the switches, DHCP snooping with the ARP table tied to the DHCP server, and port security on access ports, and keep VLANs segmented so a compromised device cannot reach the rest of the network.

Key takeaways

  • ARP has no authentication built in, so any device on the local broadcast domain can claim to be the gateway for every other device on it.
  • The usual outcome is a man-in-the-middle: credentials, session cookies and form data can be read and modified without any visible error to the user.
  • Dynamic ARP inspection on the switch blocks forged replies and is the single control that stops the attack at the point where it happens.
  • DHCP snooping with a trusted DHCP server and a binding table of learned IP-to-MAC pairs stops an attacker from assigning themselves a victim's address.
  • Because ARP is a local protocol, the attack is a local network problem, which is why segmentation and switch configuration matter more than a firewall on the perimeter.

As of 24 October 2026: the service descriptions, prices and turnaround figures on this page are current.

What does an ARP spoofing attack actually do?

It works because ARP has no authentication. When your laptop wants to reach the gateway, it broadcasts a who-has request, and normally the gateway answers. In a spoofing attack, the attacker's machine sends unsolicited replies claiming the gateway address belongs to it, and the victim's machine updates its cache and starts sending traffic to the attacker. The attacker's machine forwards that traffic to the real gateway, so everything still appears to work, but every packet passes through the attacker's machine on the way. From there the attacker can read plaintext traffic, capture cookies for a logged-in session, or modify the content in transit, such as changing the payment details in a form before it is submitted.

Which switch settings stop ARP spoofing?

Three, and they work best together. Dynamic ARP inspection validates ARP replies against a table of what the switch believes the correct bindings are, and drops anything that does not match. DHCP snooping marks the ports connected to your legitimate DHCP server as trusted, and every other port as untrusted, so a device cannot forge DHCP responses and hand out addresses. Third, port security limits how many MAC addresses a port will learn, which stops an attacker plugging in a laptop and claiming several identities. On managed switches, enable these on every access port rather than only in the server room, because the attack works best from an ordinary desk.

How does segmentation reduce the blast radius?

Because ARP only works inside one broadcast domain. If your finance laptops, your CCTV recorders and your guest devices are on separate VLANs, an attacker who compromises one segment cannot poison the ARP tables of another. Put CCTV and IoT on their own VLAN, because those devices are the least maintained and most likely to be the entry point. Put servers on a different VLAN from staff desktops, and enforce the separation with access control lists on the switches rather than relying on the design intent. When an incident does happen, a properly segmented network also means the logs you need to investigate are on far fewer devices.

The main controls against ARP spoofing, and what each one actually blocks
ControlWhat it blocksWhere it lives
Dynamic ARP inspectionForged ARP replies claiming another device's addressManaged switch, on every access port
DHCP snoopingFake DHCP servers handing out the victim's addressManaged switch, with one trusted DHCP server
Port securityOne port learning many MAC addressesSwitch access port configuration
VLAN segmentationA compromised device poisoning other segmentsSwitch and firewall design
802.1XUnauthorised devices joining the network at allSwitch and identity provider
Encrypted transportTraffic being readable in transit even if routed wronglyHTTPS and VPN on client devices

What are the frequently asked questions?

What is ARP spoofing and how do I prevent it?
ARP spoofing is a man-in-the-middle attack against the address resolution protocol, which is how devices on a local network map an IP address to a physical address. The attacker floods the network with forged ARP replies so their own machine receives traffic meant for someone else, silently intercepting and altering it. Prevent it with dynamic ARP inspection on the switches, DHCP snooping with the ARP table tied to the DHCP server, and port security on access ports, and keep VLANs segmented so a compromised device cannot reach the rest of the network.
Is ARP spoofing possible over the internet?
Not in its classic form. ARP is a local protocol, so the attacker has to be on the same physical network, or on a network path they have already compromised, such as after getting a foothold on a router or a virtual private network endpoint. This is why small office networks with flat addressing are the usual victims, and why a compromised router is such a serious event.
Does a firewall protect my office against ARP spoofing?
Not by itself, because the attack happens between two devices on the same local segment and never crosses the firewall. The controls that matter sit on the switch: dynamic ARP inspection, DHCP snooping, port security and VLAN segmentation. A firewall still matters, but for what leaves and enters the network, not for local traffic.
How would I know if I am being ARP spoofed?
Symptoms include pages loading oddly, certificate warnings on sites that normally work, an unexpected proxy prompt, and traffic appearing to be routed through a device you do not recognise. On managed switches, the reliable way to know is to read the logs for ARP or DHCP snooping drops, and to run a periodic check for duplicate IP-to-MAC bindings on the network.
Does a VPN protect against ARP spoofing?
It reduces the damage rather than stopping the attack. A VPN encrypts your traffic, so even if the attacker positions themselves in the middle they cannot read or alter the contents of that session. The forged packets are still on the wire, so the switch-side controls remain the primary defence.

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Last reviewed: 24 October 2026. Information on this page is general guidance, not legal advice. Prices are indicative bands for a typical scope and are confirmed in a written quote before any work begins.