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ARP Table

The ARP table is a local cache that stores mappings between IP addresses and MAC addresses, eliminating the need to broadcast an ARP request every time a device communicates on a network. Entries in this table expire over time to account for changes in dynamic network environments.

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About this video

Network communication requires devices to know not just an IP address but also the corresponding MAC address of the destination machine. The Address Resolution Protocol handles this translation by broadcasting a request across the network, prompting the target device to respond with its MAC address. Once that exchange is complete, the requesting machine has the information it needs to deliver traffic at the data link layer. Performing a full ARP request for every single packet sent would be inefficient and create unnecessary network overhead. To solve this, devices store the resolved IP-to-MAC address mappings locally in an ARP table, sometimes called an ARP cache or ARP cache table. This cached record allows a device to look up a MAC address instantly for any IP address it has already resolved, making repeated communication with the same host significantly faster. Because networks are dynamic, device configurations and addresses can change over time. To prevent stale or incorrect mappings from causing communication failures, entries in the ARP table are not permanent. Each record carries a timer and will expire after a defined period, prompting the device to perform a fresh ARP request the next time it needs to reach that IP address. This expiration mechanism keeps the ARP table accurate and aligned with the current state of the network.

What you'll learn

What's covered

ARP Table

Aligned to

Cisco CCNA
1.13 Describe switching concepts
CompTIA Network+
1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.
Cisco CCST Networking
4.5 Explain basic switching concepts
Cisco CCST Cybersecurity
2.2 Explain how network addresses impact network security

Key terms

Address Resolution Protocol
ARP
A protocol used to map an IP address to a physical MAC address on a local network.
Media Access Control
MAC
A sublayer of the Data Link layer that controls how devices on a network gain access to a medium and transmit data.
IP Address
A numerical label assigned to each device connected to a network that uses the Internet Protocol.
ARP Table
A cache stored on a device that holds mappings of IP addresses to MAC addresses, reducing the need for repetitive ARP broadcasts.

Transcript

Now, if every time a machine needed to communicate it needed to do an address resolution protocol, that could get inefficient. So there is something called an ARP table. Let's take a look at that ARP table.

When a machine needs to talk to another machine, it needs to find out what MAC address it's going to be communicating to. It probably knows its IP address and is communicating to its IP address, but it needs to translate that to the MAC address. To find out what that is, it will do an address resolution protocol and say, hey, who belongs to this IP address? And then in reply, this machine will reply with its MAC address. Now this machine has it.

But of course, if it has to do that every time it sends a single packet, then that could be problematic. So what it does is it caches that information. This cache is stored on the machine, and what will happen is it will make that ARP request, it will get a response back, it communicated to that specific IP address, so it knows that, and it will store the MAC address. Now it has that association between the IP address and the MAC address.

We call this the ARP table. Or, since this is a cache, it also could be called an ARP cache, or an ARP cache table. Whatever the name of it is, it just stores all of the mappings of the IP to MAC addresses.

But what will happen over a period of time, since this is a dynamic network and it changes, then it will expire those MAC addresses that are in that ARP table. The entries in there will expire.

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